I have two houdini hatches on my boat, which as you all probably know attract alot of condensation, and after spells of hot and cold weather tend to leak a little. After years of taking apart and resealing I came up with the idea of having some covers made out of polycarb. These sit over the existing hatch and are held in place by locking pins. The hatches still open and of course still let all the light in. In the winter I put some bubble wrap inbetween the hatch and the cover and have had no condensation this year, and the light is still there.
Polycarb as I've been informed is a difficult substance to glue. I used a silone type glue at first but it came apart after a week or so, then I tried different glues with the same results.
Anyway I've found a company that will make them out of 6mm polycarb (with welded joints) cheaper than it was to buy a sheet of the stuff to do myself.
The company is Plasbrun in Cannock 01543 462802. (I did the locking pins myself.)
I hope this is of use to someone