I've seen one hunting camper that used a plywood panel over the slide that was attached at the roof line with removable pin hinges. There was a rubber garage door seal that kept rain from leaking past the joint. As I recall, it had simple hooks and eyes at the slide edge to keep the wind from lifting the cover. The plywood was painted with high gloss deck enamel so the snow would slide off.
Dutch
2001 GBM Landau 34' Class A
F53 chassis, Triton V10, TST TPMS
Bigfoot Automatic Leveling System
2011 Toyota RAV4 4WD/Remco pump
ReadyBrute Elite tow bar/Blue Ox baseplate