My new Mini X already had a bushing in the RF, but the dogbone popped out anyways
I tried to use a spare plastic bushing, and sanded it thinner, but after pulling apart the front suspension about 7 times (no kidding) trying to get the bushing just the right thickness, I decided to use a spring instead. I used an extra Mini Z stock front suspension spring- cut down to size. A cut-down pen spring, as suggested already, would work just as well.
Works perfectly!