2 more, on the list
And intense1, the rubber diamond plate would work very well.
Couple of things though, I would probably not glue it, but rather use SS button head screws, with washers and T-nuts to kind of rivet the rubber down. This could easily be done after the fact by cutting rubber with the ply as a template, however, the rubber would be 1/8 of an inch above the lip of the frame. The other option would be to recess the lid an 1/8. And because I am not willing to compromise the strength of the ply by using a thiner piece, this would necessitate using a different sized piano hinge, and welding the lid stop lower. The latches would have enough take up to accommodate going over the rubber.
I really like the idea, I honestly do not think I thought of this before, or if I did, ruled it out for some reason. This could be done on a custom basis for a flush fit, or by the end user. I will look into the material costs. It would probably add a good bit to the price since there are quite a few steps to do it nicely, the cost of the rubber (which I will check with my rubber supplier on, SOR is not exactly famous for their good deals) and the stainless hardware.
The unfortunate thing is that the finishing on the ply would have to remain the same in order to make sure it is impervious.