After considerin gseveral possiblities, the solution I came up to was a little different.
To remove the abgle, I created a small wedge out of PVC block, and epoxied it into the curve, then drilled through it. This created a flat bottom, so now a good ol' long bolt and wingnut will work nicely.

Here's a snap of it:



Now it's all structurally done, just needs final touches and paint.
There are a few places where it's hard to reach the bolt top and nut at the same time. I think I'm going to give a try to a nutsert/ rivet nut in the fiberglass to deal with this.
Supposedly they are OK to use on fiberglass - I'll be sure to use aluminum ones, so the pressure will be less when crimping it into place.
Does anybody have any experience with this? e.g. adding using rivet nuts in fiberglass?