I would get some small stainless steel mesh. Cut out a rectangle or square for the size you need. Get a cheap Amazon or Harbor Freight plastic welder iron kit. Use the iron to gently melt the mesh into the area over/around the hole. Make sure it melts about halfway down into the tank plastic. Now you have base for a fuel resistant epoxy to grab on to and hold. Apply several layers, (letting them dry between layers), to build up to match the rest of body. Let fully cure/dry for a day or two then sand/shape to your liking.
Check Youtube for vids on how this principle works. I have used this method on ATV/dirt bike plastic fenders and such along with the stainless steel staple method.