Re: Bought a 4 stroke kit from BikeBerry
Assuming that mount plate is steel, and that the welds look solid, I'd examine it very carefully to see if some kind of forming is possible. Like maybe bending the lower part of that grooved flange outward to better contact the seat post. You may need to heat it to keep it malleable while you work it. And I'd work it over a seat post clamped in a vice to maintain proper shape.
But if it doesn't look like the plate would hold up, I'd use some kind of filler. Epoxy resin could work. Shims may work. I've always been iffy about rubber, but this is a four stroke so who knows. I've heard it done where one guy wrapped a post and mounting bracket all around with aluminum tape, sealing from the bottom up and leaving the top open. Then he dumped melted lead in a gap he was trying to fill. It cooled and set just fine, but I never got to find out how it worked over the long haul. I imagine it was fine though.
"There is nothing wrong with wanting a motorbike that is an extension of your personal taste and fashion sense; if you must ride somewhere, I say do it with style