Tanks can be a pain when trying to get a fitting top. They come in different dimensions that equal the same gallon size. What I mean is you can have a 20 gallon tank or a 20 gallon long tank. Both hold 20 gallons but the dimensions will be different.

I would measure yours and see if you can find or order the proper top. If not I would try to sell it or trade it for a proper reptile tank.