I am 6'3" 200 lbs and I use an ultra shield containment seat, I had this problem and used a large ratchet strap to bring the sides of the seat in as tight as I could stand it. I have to turn my body sideways to get into the seat, and now my arms don't even feel the side of the seat.
Also I saw you guys posting about earbuds, I made custom form fitting earbuds with silicone (not silicone 2) and cornstarch, mix equal parts (use rubber gloves - its easier) it will feel a bit like playdoh, coat your ears a bit with the cornstarch and insert your standard ear-piece , then pack the mix into your ear - wait about 5-10 minutes for it to harden. Just like the custom made ones but only about $2, there is a down side to these though, you wont hear when a car is next to you. But you will hear everything on your raceciever.