Hey DD,
I use fast setting, two-part epoxy to create flares right on to the bodies. It's readily available locally (JB Weld brand), strong and easy to file and sand.
My procedure is:
1. Glueing in a support platform in the wheel arch. I've used thin styrene previously, but on the Bugeye I tried masking tape which I like better and will use in future projects.

2. Dabbing in successive layers of epoxy until the final rough shape. For the Bugeye, I wanted to have close to as matching flares so I measured out distances and marked them with masking tape for references and limits to apply epoxy.

3. Filing, sanding, puttying and priming OVER and OVER!!

It certainly takes some patience, but worth it in the end.
Hope this helps.
Dan