Oh Si, lend me you ally welding gear
Right getting realistic just for a moment, whilst others may have funds to burn, I'm guessing you won't want to be doing this more than once and at the most cost effective.
to that end, create a template,rough shape you want and showing the mounting holes, to do this get something like a metal biscuit tin lid (you should be able to cut it with snips (or a big pair of scissors, but don't tell Jo
), you should be able to try out various angles of bend to get the right ones.
Once this is done, if you want the shiney look get some stainless, however mild steel will be cheaper and if you're painting it it'll be just as good, 4mm sheet will do nicely and if you bring the template and sheet to me I'll fabricate it for you.
If on the other hand you want to do it yourself, then probably 2mm will do, but make sure you vector in either a central mounting point or one either side to fit a brace(s) to support it, given your access and tools, I'd drill and rivet it to the main bracket.
Cheers Mark.