I'm sure this will have been covered before, what hasn't. I need some rear arches 10-12 inch wide or a way to make the ones I have wider. Any cheap sujestions?

I made my rear arches from skinny ones I bought from an MG man. I cut them in half and opened them up to the width I wanted, then rebuilt with resin and glass.

I used a jig made from MDF. I think there's a picture in my archive

That is a good idea, will have a go and see if it works on my rears.