I also fly gliders and have repaired a carbon boom or two. The best way to fix it is to feather (sand thinner) the carbon towards the break until it is barely there at the crack. Then, get some laminating epoxy(which is different than the epoxy you would normally use) to wet out some replacement carbon fiber. I like to wet out the fibers first and then squeegee out the excess and then apply to the area. Wrapping with old VCR tape or the backside of electrical tape can be used to tighten them up and hold them in place. The laminating epoxy is very thin almost like water and penetrates the fibers very well. Two part stuff just won't cut it. Laminating I think is also stronger since the only filler is the fiber you are bonding not junk to make it thick. Often it will take overnight to setup and sometimes, depending on the epoxy, a few days to fully cure. Hot boxes, hot attics or being left in the car in the sun will often cure it faster and harder.

IMHO,
Frank