To square up a crack in my RT fairing, I used a piece of wood and some sheetrock screws to make a splint. My Nurse wife told me the medical phrase for that is an, "external fixator".
After the splint was in place on the outside, I applied Bondo fiberglass jelly on the inside of the crack.
My plan is to sand down this first layer, then add some fiberglass cloth, then more jelly, to strengthen the repair.
Since I'm already going to re-paint, filling and painting the holes left from the screws will not be that much more work.