Good questions... there is a silicon sealant that will adhere plastic to plastic, so I plan on using that.
I plan on using a 1/2 in thick piece of PVC, and it will be supported in the middle by the baffle in the tank, which comes to the top about mid way across the tank... there is a gap of about 4 inches between the end of the baffle and the tank wall. The actual piece will only be about 30 inches wide, and about 45 inches long, with that 45 inch dimension being supported in the middle.
I could do the whole thing in wood again, but I feel that the one piece all plastic PVC cover is probably vastly more "water resistant."
As far as ribs on the "deck" above the tank cover, I plan to have these with gaps or "mouse holes" in the ribs, to allow for air flow. Something like my crude drawing below...