So I decided to try a different direction... I drilled my bottom holes a little wider, and drove some T-nuts into the plastic. So if the T-nuts hold up OK, I'll be able to install/remove the bottom fins without unpacking anything.
I'm not sure about how likely the nuts are to wiggle out on their own, while I'm swapping the fins. So this weekend, I plan to add some CA glue -- and then we're gonna take it out Monday for a few miles to see what happens.
I'll let you know how it goes -- maybe with some pics, too.