If ya could, I would try to reinforce the inside with fiberglass, give it a good base, so things don't shift around, next trim the loose pieces, then build up that area with your material fo choice. If you try to patch the area without reinforcing it, after a while the tank could "break" loose and the area would crack. You will probably need to reshape that area once you build it up, but build it up in layers.
Well, I saw a couple of people recomend JB weld, plumbers putty,and bondo. I would start with fiber glass on the inside, and make a good solid area to build up on. I would then build up with the jb weld, until you are almost to the point that you don't need to fill anymore, then just fill the rest with bondo. If you use too much bondo, it is somewhat soft and brittle, so if ya use the jb weld to fill most of the area it will be less likely to crack, but bondo is very easy to sand, that is why you only want to use it to "even" out the contured area. Get rid off all the loose pieces of plastic, they may show through the patched area and are hard to hide when you sand and paint. Just a suggestion