This small tubing fits so close with such a small gap that I didn't bother notching it. I just filled the small gaps with my welder. If I needed a tighter fit for some reason, like maybe TIG welding, I would have used my Harbor Freight tube notcher. I used it extensively when fabricating the frame and radius rods.

Fiberglass can do weird things. If it's pulled out of the mold too soon, it will warp; if it's left sitting unsupported for a short time, it will warp. The warmer the ambient temperature, the faster it deforms... Years ago, a friend of mine bought an old Anglia gasser body that had been sitting for several years in a storage shed. The body was all steel except for the one-piece tilt doghouse which was fiberglass and had been sitting for years with boxes, etc. piled on it. When they pulled it out of the shed and set the frontend against the body, the fenders were so deformed it looked like it was about to take flight. Needless to say, the one-piece frontend got trashed and replaced.