One thing I've used is chewed chewing gum. Some of the stuff that is "long lasting" or known for not turning to chalk after chewing it after a few minutes. It'll get pliable with heat, but isn't goopy, and isn't 100% solid when colliding with other objects on the track.

Did this successfully with my M03 and a new diff. I don't race much, but it did lock it up very well. Had good success with Trident brand (Because that is what my wife chews)