This is something I thought of too 😀
It's one of the things I spent time on since I got my 3d printer a couple of years ago. I've successfully fit a 486 and 386 baby AT motherboards into two different mATX cases.
You can see pictures of my adapters here: Re: Bought these (retro) hardware today
My 486 case is an unknown make full-height mATX desktop case with an SFX PSU. I bought it specially because I could see that unlike many mATX cases, the standoffs could just be unscrewed, so adapters can sit much lower with no risk of screws causing shorts.
My 386 build is in a Yeong Yang A200 case, which was my reference 3d model as someone shared very detailed design files for it. It has pressed in standoffs and works great with the 3mm 'riser' to stop the board shorting on the standoffs. Somehow the case was sold with no front panel, so of course I made one with the 3d printer and made it a desktop instead of a tower:
The files can be had here if you've got a 3d printer: download/file.php?id=92248
You can make some adapters that sit the motherboard 3 or 4mm higher than the original standoffs, then there's a separate riser for the card slots. I recommend to print them in PETG since PLA can warp at quite low temperatures.
The zip file includes:
- The backplate for ATX > AT with that micro ATX position, which has a built-in card slot for PS/2 / serial / parallel ports. That secures in place with a square m3 nut that presses into the screw hole to make a strong thread.
- An adapter to raise up the card slots mounting surface in-line with the mainboard's new height. This will need longer screws to secure the cards.
- Front & back mATX to AT adapters. The screw threads designed into these are UNC 6/32 fitting.
Beware that the adapter bracket screws attaching to the original standoff locations might be very close to the back of the board. Use small hot swap caddy screws (UNC 6/32 countersunk) if the original risers use UNC 6/32 (coarse thread) or countersunk M3 screws (fine thread) . And / Or put insulation tape on top of the screws.
For the ATX power, I just use TFX or SFX power supplies with an ATX > AT adapter, sometimes with a -5v adapter modded in. My power switch also is just converted from a regular momentary switch to a latching switch that connects into the green + black wire of the cheap ATX > AT adapter cables.