I would begin making the enclosure secure by constructing a wooden frame around the stock tank. You could easily purchase some 2x2's from a hardware store and create this frame around your cage. I would then cut plywood to fit the top of the frame and secure it with screws. Using a jigsaw or circular saw, you could cut the size opening you would want for the lid. I would make the lid open on hinges and secure it by barrel bolts. You can find all these products at Lowe's, Home Depot, or your local hardware store. I would stain and finish the plywood with spar varnish so you can make the wood water-resistant and easy to clean.
Approximate cost:
2x2's= $1.50 each
4'x8' plywood= $15.00
Hinges= $2.00 for set of two
Barrel Bolts= $1.00 each
If you have any other questions, please e-mail me.
I have created cages in much a similar fashion.