When the tank is empty, dry the inside of the tank where the hole is with a scrim. If you can reach it.
Then put gaffa tape around the hole whilst your tube is in position. Tape over the hole as best you can. Then carefully from the outside of the tank, use expandable filler. Once dry, use a knife to level off.
Thats what i did anyway and dont forget to remove the gaffa tape from inside the tank once dry as well. Do this first before you hack away on the top side.
Or if the hole isnt too big then do the same but with a hard setting/water proof tape setting thick glue that wont fall straight through the hole and then sand smooth whilst catching any debris so it doesnt contaminate the tank. (If that makes sense)
But others may have some other suggestions to help as well.
