ever heard of laminating? they have laminating machines in graphic stores. if you are talking about other kind of sukkah products then let us know so people can try to help their loving brethren.
Most people are going to answer to “laminate” them but your latest response seems to indicate that you don’t want to do that. What sort of “priceless decorations” are you talking about?
Perhaps try shrink wrap bags and mount with double stick strips? (I never tried it, hence the question mark. Jewish party goods stores carry them for mishloach manos.)
Or if you are ready to spend try finding a water proof shadow/display box. Or perhaps you can caulk and seal the gaps of a regular wall-mountable display box. Make sure the depth is enough for your stars. If you use a “shlock” you might not even need to seal it, but of course that’s your call.
Buy a shlock……..except if it doesn’t rain to the point that it ruins your soup you are still required to eat in your sukkah and therefore the decorations can still get wet.
The way my I’ve seen these waterproofed is by pinning it to a cardboard (yes, this will make some small holes in these priceless stars) and then covering it with heavy plastic. You can buy this by the yard in some places.