some one else aske about the lid on anothe forum so i copied and pasted my answer for you guys sorry about the typos i was in a rush and now im to lazy to correct them
well i'll try to explain quickly because im off to drive a van to germany in a few minutes, i used arcylic (sp) the sheet cost me 12.99 from wickes building supplies. you also need a very sharpe knife and some silicone. you can't cut this stuff its to thick you have to score down it with your sharpe knife then snap it. so you make your top and 3 or if you want 4 sides lay it all out flat and using masking tape turn the sides into flaps then fold them up and join the corners together with more masking tape. to get a nice even bead use masking. and then squeez your silicone around all the joins then run a wet finger over it to give a nice finish. sorry about all the typos in that post but i just im any minute now laters...................... be warned it can sag in the middle so some small supports could be a good idea. or use perspex instead
ps holes for vetalation is a good idea to