will sound strange ... but you can do a small bondo patch
around the spot ya wanna fix... say an inch and a half wide
make sure the aluminum is scratched up or has a rouch surface first
then apply your layer of bondo but not too thick,,,
just before it sets up... criss cross it with a razor knife
making a grid good pattern
/X/X/
Now you have a "
RESIN" based patch and you can fiber glass
right "to that" with resin cloth and or wood layers as needed making it stronger / thicker than the original aluminum......
i have even repared gas tanks this way and it worked fine.