What kind of nest would you recommend for a Formica subsericea colony w/ ~25 workers?

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Greetings, users of this website that I apparently only use once a century to ask stupid questions! I'll make some ant journals eventually, I swear. My larger colony of Formica subsericea has been approaching (or has already reached; dunno exactly how many workers they have since there's always some hiding in this groove they've made in their water cotton) the mark of 25 workers, and they are, in my opinion, outgrowing the "single test tube in a food storage container with some sand in it" setup I'm currently housing them in. I could just add another test tube and make an official "tub and tubes" residence, but I was thinking about getting a true nest for them once they reached 25. I've been browsing the Ant Keeping Depot for options, and, well... quite frankly, having no prior experience with this, I've no clue what would be best suited for them. I don't know what kind of nesting material they'd like, for one, but mainly, I'm concerned about size; it's one thing looking at the dimensions of a nest on a webpage, or even measure out a space on my shelf to accommodate said dimensions, but it's another thing entirely seeing it in person, y'know? I could order a product originating from halfway across the planet, only for it to arrive and find out that I miscalculated and it's too big or even too small. So, in part due to my perpetual anxiety that something will go wrong if I do something for the first time without assistance, I'd like to ask for help.

Don't worry about price too much! I have plenty of excess savings to spend at the present time. Also, sorry if there's already a topic along these lines.

When it comes to nests you need to decide what you want the nest to do for you and for your ants.

Do you want to be able to observe the queen and larva?
Do you want to see natural digging behaviour?
Do you just want the ants to be happy and thrive?

Since, like me, you are newer to this hobby I'll assume you want a more visible nest. I also have formica subsericea and I have them in a "natural" set up. My colony is about 80 workers but I hardly ever see them. As formica colonies get larger they can get very skittish. I was worried that in their old nest they were being to stressed out by natural incidental changes in light so I put them in this set up to help them thrive. I've seen glimpses of eggs and the food I leave disappears so I think it's working, but part of me wishes I left them in the old nest. Here is what they currently have:

The nest building you see the ants doing in this video lasted for about 3 days. It was wonderful to watch...but now I keep the sides covered and peek once a week in low light, they have arranged the nest so it's hard to see, so I think I'd go with a different design in the future. I made the "mountain" myself, and I think I could do a better job of carving spaces for ants in the future. The enclosure is from "antcube" their products are wonderful, but expensive for us to get in the US... even if you aren't that concerned about cost. I'm talking $80 shipping fee expensive since you will need to use a reshipping service. (if anyone can tell me where I can get glass cubes like this in the US I'd be so happy... everything I find is made for fish, too big, does not have an ant proof lid or is just ugly)

Before that I kept them in a nest like this from "aliexpress" it's quite good and probably the first one I'd recommend. (though the suggestion to grow the colony more first is a good one) this species can benefit from a large outworld:

This nest is nice because you can block two of the chambers via ports in the nest with long tweezers without removing the lid. My Pogonomyrmex are now in this same nest. It's sturdy and reusable. I kinda wish I hadn't moved the formica colony since I could see them so much better in this nest compared to the natural one. I just got worried because the queen stopped laying eggs.... I thought it was the stress of the light and not being able to dig. But on reflection I think she was just taking a natural break. You live and learn.

(Just a note about "blocking chambers with cotton" DO NOT do this if you have a plaster nest. The cotton will soften the plaster and you may need to deal with an escape attempt. This method is fine, however in a plastic nest like this one. You could even block 3 chambers if you can manipulate the cotton far enough in. this would make the nest small enough that 20 ants could be happy in it, especially those as large as this species. I blocked off 3 chambers and moved my formica in with 18 ants. They grew to 50 shortly after. I removed one of the cottons and they grew a bit more. Then the queen stopped laying eggs and I kinda panicked and probably overreacted by moving them to their current "natural mountain" nest. It might be better to just leave them in their test tubes. But I simply connected the tube to the outworld and they moved in one day. )

The other nest I'd consider is from "underground cities" on etsy I keep my lasius ants in this one and for small ants it's amazing. I'm curious how formica would fare in something like this. I recommend replacing the plastic dome with a 50mm round piece of glass which you can buy by searching for 50mm lenses on aliexpress. The plastic domes aren't my favorite part of this set up. But the simple round shape is great. You do need to keep a sharp eye on hydration. Are the type who forgets about your ants for a few day this is not for you.

This is how it is replaced with a glass lens.

I'd recommend adding a bigger outworld for formica since they like to send out ants to roam and it's fun to watch. I've been increasing of the opinion that many formicariums are just too deep.

Ants can really pack in to small spaces and they LIKE to live that way. Look at my lasius colony now.

In the same space! and they still don't need more room...

I think one of the hardest things to learn as a new antkeeper is, and this is funny when you think about it, that ants are ... small. Not only are they small but they live at densities that offend our mamalin sensibilities.