Crease it all you want, it will still crack. Shit, 18 gauge won't even survive on a swingarm bike--my friend's sportster had a taillight mount that was well constructed by him out of 16 gauge on his sprung sportser and THAT cracked. So a rigid triumph will destroy his bracket for sure.
I built my first license tag for my evo out of 16 gauge and I lost a license plate maybe 3 days later.
My general rule of thumb is, figure out how strong you need something to be and then make it twice as strong as that (at least on a rigid bike). You'll never regret it.
Put enough creases and bends in it you might get it ridgid enough, but using a thicker steel would make sense, and probally be worth the efforts.