They may not be hexagonal but there are items available called rivnuts or rivet nuts. They are applied using a tool not unlike a pop riveter into pre-drilled holes.
However, many folks just use plain hardware with shakeproof nuts - no particular need there for a rivet nut as the back of the wing is easily reachable to put the nuts on the bolts. it's not like you'll be taking them off on a weekly basis, after all.
Alan