Is there any point going to all that trouble if you've got other holes rusting through? Edit: never mind, I realised it was a before and after picture...
I used 16 gauge (actually 1.44mm) as I recycled a piece of office door nameplate. Don't think it's critical so anything around 1 to 2mm would be fine. Thicker is harder to cut with tinsnips but easier with a jigsaw.
I wouldnt replace it with a Westfalia hole off an early 50's Splittie. It will cost more than the bus is worth.
I made a plug out if two oversized bits of black vinyl flooring. I glued them together in the centre, and pushed it through like a grommit. The vinyl came from a shop sample and was free! Sent from my ONEPLUS A6013 using Tapatalk
You will be getting close to vinyl plastic melting when the bus stops on a hot day. Expect a sad floppy creation one day..