Often of thick material.

They sell those by weight vice their standard sheet prices, so if you find something in the material you want, it can be a good deal.

This is a 70 gallon tank and the opening is about 30 inches wide at one end, tapering down to about 14 inches, with a length of about 4 feet overall.

A 48 inch X 48 inch 1/2 thick sheet is $276.79 for the full sheet.

Of course the overall length might be the factor that kills this... if it is just over 4 feet, then the price of the sheet doubles, and leaves a lot of scrap.