O-ring or seal.

It may work. The medusas are open in the back; 4 large holes and three small holes. I've run my Rustler with a Medusa offroad briefly, mainly in some dry and dying grass with a few dirt pockets here and there, and I haven't had any problems yet with the holes. It may be a good idea to seal the rear if you run in an area that has lots of little smaller pebbles though.