2nd alternative....Now I tried my best to draw it out for ya w/out having the proper software at hand right now but I'm hoping you get the picture.
Buy a ticker sheet of plexi-glass.
Cut 1 circle out at least 1" wider in dia then the actual hole you have and 1 circle the exact size of the whole.
Glue both parts together.
Duplicate this so you have 2 of the same "plugs".
Glue them to the tank wall from the inside and the outside of the tank and glue all of the parts where they meet at the existing hole in the tank together real well.
What I'd do if you tank wall's are 0.5" thick I'd get half the thickness as plexi glass, building 2 of these plugs will then fit flush into your tank.
I'm not good in explaining DIY stuff am I!? I hope you're getting the picture .....