A motherboard tray to hold your motherboard? Are you just going to glue it to the side wall or keep it flat at the bottom?
Nah, people have run machines in mineral oil for excess of a year without the fans even slowing down.I'm not sure the fans will like being run submerged in oil, however good they are. They'll probably be ok for a while, but since oil is a lot more viscous than water you'll be putting much more strain on them than they were designed for, so I really wouldn't be surprised if they burnt out and packed up in a while!
Will doOh yeah I forgot to mention.
Use cell cast acrylic, NOT EXTRUDED!!!!
This is very important if it will be holding back any liquid, as extruded has a tendency to be flawed, which can cause it to break later on.
Just don't use ultra-thin plexiglass and it will be fine.
People use it to build very large aquariums all the time.