You're probably going to want an old-style ATX case, because, for one thing, most of the modern ones don't have the AT mounting holes anymore (which can be drilled, but it's a hassle), but more importantly they are also usually designed such that the 3.5" drive bays are in the way of longer motherboards and expansion cards. I have a Coolermaster Centurion 5, which still has the mounting holes (I don't know if all versions of it do), but it won't fit long cards... even the average VLB card is an issue with hard drives installed.

Unfortunately, those older style cases are a bit hard to come by new, since nobody seems to want them anymore. Really the best place to get such a thing is to find someone getting rid of an old PII/PIII ATX system, or ask any mom-and-pop computer places nearby if they have anything of the sort stashed in the back. A good way to judge is, if it's beige, chances are good it'll work with Baby-AT boards.

You've just jogged my memory about something! If you look at this photo here, it shows an Antec solo case. I had one once, and fitted a PIII-based mobo inside it, and then added in some large cards such as an AWE32, and also a GUS. It worked, but I had to slice through the drive cage area with a junior metal hacksaw.

After cutting some sections, I then bent back the broken metal to make room for the cards. OK, it looked a bit of a mess inside, but once the "side door" part was back on, these mods were completely invisible. I've just looked up this case online, and I'm amazed at how much they have gone up in price. Approximately double, in the last 18 months I would say.

This mod would work for me, because I only ever use 1 HDD. And I usually put it inside a 5.25" HDD cooler, which can go in the area above the entire HDD drive cage area. Also, if you examine that photo mentioned above, it seems to have a lot of mounting holes. Unfortunately, when I did have this case, I didn't have an AT mobo at that time, and so I don't know if it would fit.

For you I would recommend a decent test bench!

Obviously it needs AT mounting holes, but I'm sure it's possible to rig something.

Personally what you are doing is fine. IMO there is no need for a case if you swap stuff all the time, it will just piss you off after a while 🤣

Unless you are planning on building one machine that doesn't get touched.

Something like this:

http://www.pccasegear.com/UserFiles/ll-t60a_l.jpg

That test bench looks really good! Actually, I was planning on building some machines that won't get altered at a later date.

I never actually checked for myself when the AT mounting holes were discarded in ATX cases. It will also depend on how much you'd be willing to spend on it, since shipping a case won't be cheap. If possible, you could opt finding a case locally (one way would be to just buy an old P3 and "hope" it's got the mounting holes).