Well, you would have to calculate the surface area of your case (and brackets and such) to find that out. And know the size of the HD sheets. It's a surprisinly large amount you need for making only a case...
Home Depot makes some very big sheets but I owul dnot reocmmend doing that... plexi is better in smaller sheets because you have more straight edges and if you ruin one sheet the whole thing is not lost... Large sheets of plexi run 50+.... not cheep No you can not do a whole case onan 18 x 24 sheet...