The hooks just help you align/assemble but don’t actually support it, the bolts along the front lift it up from the hooks.
If you can weld then 18 gauge should be easy, even with a stick welder, so long as you can turn it down, below 22 becomes a challenge

if you go thicker then the forming becomes an issue, even if you have a fly press or similar.
Rather than sacrificial anode, could use galvanised sheet, the zinc is the sacrificial. Plug weld it to the frame and use a zinc rich galv repair paint. Works on the farm gates. Will measure up tomorrow and decide to patch as mild steel or buy a sheet of galvanised. Nibbler would slightly wipe the edges with the zinc. If it was produced commercially in small batches, would be laser cut.