best would be to "solder" a patch from the same type of plastic as the tank is made of.. probably high density polyethylene (spare gas tanks are made of that)
Look on the bottom of a plactic container for the recycling mark.. should say HDPE .
fiberglass, like Bondo, is polyester.. gasoline would dissolve it in a second. Epoxy-glass would work but is too expensive. Since the tank is very flexible i wouldn't count on other epoxies, like JB Weld to last long.
paper gaskets are an option if you want do not want to permanantly plug the hole