Not sure what you're working on, but it is often easy enough to bond a strip of aluminum, or small pieces of aluminum, to the backside of your fiberglass part. Not only for clecoing, but to give your rivets something suitable to bite into.
I don't know about others, but it seems that my fiberglass work always involves grinding/sanding down those flange thicknesses to get a good fit. Those thin spots seem like a good place to have a bit of extra meat, i.e. thin aluminum to back it up.
Any of the tip fairings, including wingtips, seem like a suitable spot for this.
ShoeGoo glue seems to work well to attach the aluminum. Proseal, or any other flexible glue should be OK. Epoxy resin doesn't stick as well, but can be used too... just don't push too hard on that drill bit or cleco.
I also keep a stash of aluminum washers handy. Interestingly, they can often be found in the aluminum gutter section of the local aircraft big box store. Or just go to online behemoth Amazon and search for 1/8 aluminum washer
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