No shit, huh? I pray to God that this wasn't on a Benz or a BMW...Fix it correctly, repair professionally or replace it.
I know if I dropped the tank in a car and found that some person had drilled through it then fixed it halfass with silicone and screws I'd be pissed.
This is why I'm mortally terrified of used cars when I don't know the previous owner.
I mean hey I made my fair share of install mistakes and I know the feeling.
I'd get it fixed professionally. I don't trust epoxy would stand up well to gasoline in the long term. You also should do something about it fast. If you have any power cables near those holes, and they just happen to short to ground, well let's just say things ain't gonna be pretty.