If you happen to have an in-ground concrete pool:
Drain it, then stack truck tires all around it, creating walls. Fill them with dirt, sand gravel, whatever you can find. Use powerline poles or timbers as rafters. on top of these place plexiglass or plywood with a large tarp covering the wood, then cover with 36" of dirt.

A lot of in-ground concrete pools have a ramp, which will be very convenient for you while stocking it with supplies. Use a hand truck or wagon to make things easier. If you have no ramp, you can make one using cinderblocks and a lot of concrete mix.

On equipment: You get what you inspect, not what you expect.
On training: Our drills are bloodless battles so that our battles are bloody drills.
On tactics: Cheating just means you're serious about winning.