Figured as much.Absolutely.
I've used a number of Items. My favorite is ABS (Black pipe), it seals perfectly and once your force thread a brass barb into it it's leak proof. I use some glue around the barbs just for extra assurance against a leak.
Other options are plastic containers from a dollar store, but they take more rigging to make work.
Use your imagination.
Heat won't be an issue as it will be all insulated except for 2 panels which will allow the fans to bring air in and out of the bottom enclosure.I agree with MpG.
Thats a tricky thing with some reservoir models so keep that in mind.
The Ideal thing would be to have all the water cooling components on the top of the case. My first rig I ever made with water cooling had it all on the bottom, and it was a really pain getting the air out of the system and on top of that heat rises, just a tip.
What style reservoir do you have in mind? (Square, cylinder, funky shaped, etc)
ABS pipe does wonders so that be my choice.