When I was learning how to snowboard
I cut a piece of ensolite (the 1/2 inch thick blue padding you put under your sleeping bag) into a shape I could stuff inside my pants. Simple, cheap, effective protection from falling.
It would make your seat waterproof and warm.
Growing up racing in New England we used fisherman style rain pants: high bibs, very loose so we could get them off and on with our boots on. Sometimes deciding whether the rain was worse than soaking in our sweat was a tough decision.