that stinks. there's another thread around here somewhere talking about it. plastic would be nice but it will cost you. most cheaper plastics will sag over such a wide span. fiberglassed wood isn't perfect but it usually fits the budget. you seem very resourceful though...one method i'd like to try.... resin soaked, vacuum bagged foam board. supplies would be cheap but you need somewhat special tools...vacuum, etc. might take more than 1 try too, lol.
reversed hatch would be nice. maybe shocks to hold it open when you want? don't want the wind opening it of course. but, would be nice to have 2 hands for a change when reaching in there.