What's a good rack mount chassis? I've got a server in a cupboard under the stairs, but there's not much room in there and I reckon I could fit a 4u rack mount into my existing cabinet

If you're looking to save a buck, I bought the Mitac mch-202, which were built in great numbers 20 years ago and are still available now for about $30 usd. It's a passive backplane system, but I took out the backplane and 5 minutes with a jig saw it now houses my dual athlon mb. It's got all the usual mounting holes for new motherboards, but just not the holes for the connectors for mouse, etc... It does have about 20 slots, tho, so it's easy to remove the slots where these connectors go. The thing I liked most about this case, other than it's cheap, is that it's siginifcantly shorter, front to back, than most cases.

These are at all the local surplus shops, so you may have them there, too. Or at least look at the old rack cases they have, I suspect that all are easy to convert.

The cheapest good new rack case I found online back when I was looking was about $175 usd.

We've currently rackmounted some stuff thanks to the IT department chucking out ancient servers which we've managed to fit ATX boards in. Old rackmount UPS' make good donors too.