i cut mine with the tank still inplace. I used a sawzall at approximately this angle

And a jigsaw with a piece of 5/8 and 3/4 ply to keep the blade from hitting the tank. Special care needs to be taken as there are hoses that you could cut if not careful

Caulked and sheetmetal screwed in place. Not the prettiest thing, but should fill my needs. i used a piece of floor from another rig (and the other pieces i cut out welded to it for more strenght) as the panel. Its roughly 12 by 9 inches