As your typical bedding material is loaded in compression while shooting, the fiber reinforcement contributes very little to the strength of the material. It's main part is a as crack arrester, and for that the steel particles in the JB weld work just fine. If you can get your hand on a piece of carbon fiber fabric and cut it into 1/8th inch length short fibers, that makes a great random orientation reinforcement (wear a dust mask!). Grinding it in an old coffee mill works great too.
F 135 - the right choice