what can I use to cover my reservoir?

Theres only one hydro store in my area so they charge 3 times for everything so I don't want to buy a lid from them ($89!) so what can I use. I have a recirculating hydro drip system so the waterpump pumps the water over the plants, on to the flood table and then back into the reservoir. On the first two pictures I pulled the reservoir out so you can see the water draining back into the reservoir near the back of the reservoir but it actually drains back into it near the front like in pic 3. Im just mentioning that because any cover I use needs to have an opening for the water to drain back in. And the other pics are just of some of my buds at a little over 4 weeks flower.

A sheet of plexiglas would be cheap and you can cut and drill it anyway you want with regular tools..

"For sale" or "To let" signs. They are made out of correx, the same stuff used for nft covers. The ink usually scrubs off with a green pad.

That res doesn't look too big go look at the plastic lids at your favorite home repair store, they cut easily so you can make the drain anywhere you need it.

plywood ,styrooform board,cardboard , tin,tarp ,flat screen tv and the list could go on and on