Basic idea is to use A+B silicone putty (with an intact master bought or borrowed) to create a mold that can be clamped over a hogged out or broken area.

The mold clamps over the bad area, and then the void areas recieve a viscous filler - JB Weld, some of the 5-minute epoxies with microbaloons, etc. to prevent seeping at the edges.

Then, walk away. A long way. Very far away...to let the filler cure completely. Impatience to congratulate myself has been the undoing of a fair number of repairs, requiring me to go back to zero more often than I'd care to mention.

After 24-48 hours, off comes the mold and the wheelwell/pillar/roof/cowl is restored. Sanding/blending is often requried.

Success in this comes from starting small and working up, learning how to shape the mold so it will 'lock' in place, and how to fill without air bubbles, something that continues to bedevil a few of my older repairs.

I had very good success with this restoring a gluebomb Fairlane that would otherwise have hit the bin, as well as a Ventura Sprint with hogged out wheel wells, etc.