This is a heavily modified version of wesley081988 design. The original design is fine but it still allows the body to flex. My design prevents that by transferring the load from the rear body skid directly to the bumper and bumper mount. I have yet to do any real world testing but it can take my body weight (~145lbs) without the body flexing so it should work.

Notes:

I recommend a minimum wall count of 5 and an infill of at least 25% for added strength.