I've done it, didn't enjoy it.

sat the motherboard on a box, and inserted several cards in it. sat the motherboard in the case with the cards inserted still. lined up the cards to the back and secured the cards with screws. lined up the cards best i could. found a nail that would fit through the motherboard holes and diped the point of the nail in ink that i poured on a paper plate from a bic pen i cut open with scisors. Marked all the holes best i could. uninstalled the motherboard.

Found drill bits that were smaller than the diameter of standoffs like quite a bit but not to much. Drilled the holes. Found a 1/4th size socket driver, from a socket set that would fit standoffs. socket the socket in one of those screwdrivers that have 1/4 inch plug on the end. Force screwed the standoffs into the holes by applying plenty of pressure.

This may not be the right way to do this but its how I did it about 16 years ago. I have never did it again. 🤣

Many older ATX cases supports AT motherboards and psu-switches without any modding at all.

The last time I did anything like this was a while ago now but I had a baby-AT motherboard with AT style power supply in an Addtronics 6896Ab. Everything needed was supplied with the chassi and the power switch was a standard model that was easy to find.

Yeah, there was a transition period in case design from AT to ATX where both mounting systems were available in ATX cases. I'm not aware of recent designs having this.

I'm not using an ATX case due to not having any AT ones, but rather due to wanting an AT board that could host all the expansion cards I want, and I haven't come across any AT cases up to the task.

So far, I haven't come across any positive experiences. Another consideration is that certain ATX cases seem suitable for ISA riser boards to be hosted if the case is modified appropriately. Thereby increasing the number of expansion cards being hosted (10+) without using an industrial board and case.

I have put my 386 into an ATX case. Using a modular ATX power supply. Didn't have any problems mounting the mainboard, but had to use a blank ATX shield and drill a hole for the DIN connector. I would rather have an AT desktop, but this is what I got.