Rain? Pffft... I can't even see the front of our Freightliner under the snow!
I also replaced the foam in our Bostrom driver's seat and it made a huge difference. But, I did have to modify it a bit to fit the seat cover on the base. On ours, the cover had a flap sewn in about midway back on the base, which helped keep the base cover from flopping around and sort of tucked down tight in the middle of the seat bottom. That flap wasn't long enough to work with the new foam, as designed, since there's a rod that slides across the foam (and through the flap segments) to hold it in place. And the new foam was designed slightly different. I ended up using parachute cord to tie it in place.
Good luck with the rain, guys!