I managed to put together a tray, from one of those dead CD-ROM drives, and by trimming off the back of the old PSU case.
It was a long cutting job (hack saw), but three rivets later, and minor adjustment/trimming
and everything lines up.
Perfect fit!
I'll have pics next week.
The next step is to mount some kind of bracket, to support the rear connectors. Then I'll re-use the CD-drive's plastic sheeting, to make sure that the PSU PCB is completely protected from the tray (preventing shorts). I ordered some mounting posts to make it all happen.
I'm going to have to put the watercooling aspect aside, until I can source the perfect connectors, or decide to make that compromise, and make it all permanent.:shrug: