so, I'm looking to fill my F4i tank with water, cut it and take it to a welder to weld a plate with a lip on it. did any of you guys weld your own tanks ? I don't know which thickness of steel should i get for the plate, how to make a gas cap and what to do with the pipes inside the tank. Also I think I'll need to make some braces so it won't bend when I sit on it.
end result should be something like in the pic.

any tips will be helpful, thanks