My PSU hasn't arrived yet, but I plan to put it underneath the metal baseplate of this iMac. 

Problem: There isn't really enough space in terms of height. 

Solution: Permanently glue the RAM access flap in place and add some holes for extra ventilation.

The holes are spaced 7mm apart and are about 4mm diameter. Mine are certainly not perfect, but they're also not that bad.

*** Update this with pictures of permanently glued plate ***

I also wanted to remove some of the standoffs that are on the metal plate, because they might be in the way of my motherboard . I read online on this guy's mod of the G3 that you can just wiggle them off. After some vigorous wiggling and smashing with a hammer I got the hang of it.

Just be careful with your hammer, don't go too crazy or you might break something...

I also finally finished the mounting plate for the IO shield. Took my a couple tries, but got there eventually.