Sorry, just throwing out ideas, I have no experience drilling into gel with a spade bit.
So I guess the small size is your problem? The ones I used to do the paddle wheel on my old boat were fine to run in reverse, but the smallest in the set was 1". They look like this, since the hole is not round, it won't unscrew when you don't want it to:
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But when it turned out that wasn't deep enough, I went and got this Milwaukee that was all one piece, only the drill bit could be replaced. You could check if they make anything like this in the size you want:
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The only other kind I can think of is an auger bit. But I think it would have the same downside as the Forstner- basically both of those are meant to shave off a thin layer of wood on each revolution. I tend to think they either wouldn't cut or would bind up in the fiberglass because they try to remove too much material at once.
Hmm, how about a masonry bit? Should be easy to find in that size, though I'm not sure how well it would cut in this situation.