Update: I have uploaded a much smaller file. My design app (Cubify Design) was set up to subdivide curves to an extreme degree, and that was generating a huge file that some people were having trouble slicing.
The original power supply on my Amiga 4000 started having voltage fluctuations which a fresh set of capacitors didn't correct. I wanted to move to a small form factor power supply, but due to the way the power switch is integrated into the original Amiga power supply, it meant either running an external switch or modifying the power switch actuator in a way which I don't think is ideal. An SFF would also cause a large air gap at the back of the case, since they are half the height of the original AT supply.
This design solves all of those problems. It provides a platform for a small form factor power supply to sit on, it provides an attachment point for a power switch, and it closes up the gap under the SFF power supply at the back of the machine. The open design and air holes on one side allow for better airflow, which should help to reduce the internal temps of the case by a large degree. As a side benefit, the supports of the hard drive caddy rest on top of the power supply, just like the original design. You could technically drill and tap a new hole for the HD mount on the top of the power supply, but I'd rather not go to the trouble and risk getting metal shavings in the supply.
I am using a Corsair SFF 450 power supply. I chose that supply because of the modular cables, which help to cut down on clutter by a significant amount.
I am also using an ATX power supply adapter/switch from Amigakit:
The printed adapter require no supports. I printed mine at .3 layer height and 20% infill, and it appears to be strong.
Once the part is printed, you'll want to screw the power switch onto the part as show in the pictures above. The metal tabs on the bottom of the A4000 case then slot into the printed part, just like the original power supply. You can then place the new power supply on top of the printed part and screw it into place. The top 2 screws of the SFF power supply match up to the original power supply screw holes on the A4000 case.
The total build volume required for this adapter is a length of 170mm, a width of 126mm, and a height of 117mm.