I am far from an expert on these but have had one for a few years. I had a local welder repair a few spots and I believe he tig welded them. If hard against the fibregalss it will probably need some heat shielding during welding.
At the back your gap looks a little larger than expected but I could be wrong. There should be a hard rubber strip on top of the rear door for when it rests on the ground and a sealing strip (soft rubber) atrached to the lid to fill the gap.

Cheers Ben