You have not told us why you need an enclosure! From you picture, you can already see you need an enclosure to contain the potting compound, so that is already a non-starter!
Make your own enclosure from thin wood paneling or plywood and epoxy to clue it together.
To use heat shrink, you will need a fairly large heat gun. Even then the tubing ends are still open.
If just to keep the bottom of the circuit board from scratching or shorting, again, a piece of any insulating material, wood, or what you have on hand will work.