I use a old floor mat from a truck.

I bought chaps that are waterproof and that has worked great for me the last two years. Knees are always dry and they don't take up any room in the truck since I'm wearing them. I purchased them from Dan's Hunting Gear (google to get the site).

Construction grade garbage bag.

You change a pair of decent knee pads from harbor freight for like $5.

If you can find them , a pair of Combat pants are great because they have knee pad inserts

Thanks guys!