Matt,
I don't get fancy with the hatch cover. I don't get fancy with an EC12, I guess. I have used G10 and CF products but found that a 30 mil Acetate works well, is in expensive and easy replaced. I get mine from Tap Plastics, a chain FG and plastic hobby outlet.
As seen on the building site, the cover receptical is secured with commercial double tape and I make several covers for the tool box to carry. Last year I forgot to bring them east and this was discovered on saturday morning at the lake. I rushed down to Staples and bought a plastic three-ring binder and made one. I am still using it and like it.
Just some other suggestions and sources...