I did it for a IIe a number of years ago. I mounted the drive under the IIe keyboard and made up a custom cable. Instead of mounting the SCSI cards DB25 on the back I ran it inside and made up a custom cable that had a male DB25 connector at one end to connect to the SCSI card. Then it went to the hard drive and ended in a female DB25 connector which mounted on the back of the IIe. The drive wasn't terminated and I used a DB25 terminator which I could remove to connect other SCSI devices.

With a IIgs there isn't a lot of room inside without blocking most of the slots. Short of finding a Vulcan power supply, you could move the power supply outside the IIgs or better yet use a PC power supply outside the IIgs. Both these would let the IIgs run cooler.

If you don't like those ideas then since you're making a custom cable anyways, I'd scrounge up a 2.5 inch SCSI hard drive from an older Mac powerbook. It'll use less power than a 3.5" drive and it's small enough to mount on the side of the power supply.

Wayne