Sitting in our coach since last fall have been new LED taillights (the Command Electronics replacement set). Those still come with the same foam gasket that the old ones use, however.
My solution: I picked up a 12" x 36" sheet of
this stuff (Amazon) (Hmm looks like people use it to repair wetsuits ! LOL). I used the foam gasket as a template and a sharp utility knife to cut out my own gasket.
Now I'm ready to install the new taillights once the temps warm up a bit (maybe next week it will be closer to 50F than 20F...).