The forend started as a splinter. If I used only Bondo, it would wind up 75% Bondo and probably be too fragile to use as a pattern. Notice Chuck extended his pattern butt with wood as well as Bondo. I started by grinding a couple of flats on the forend wood, then gluing a pine strip to each side. This gives a better frame to support the glass.
when I converted a splinter LC to a BTFE, I glued on blocks of wood and used a dremel tool to clear back the wood from the metal and then bondo'd much like Mike. If you have plenty of clearance from the wood, bondo will fill it well. Use plenty of mold release on the barrels and you'll end up with a perfect contour of them in the forend.
On the top of the comb for that Parker pattern, I glued a 1/4" wide strip of wood on the comb, ground it to the DAC and DAH measurments and used that as the target and support for the bondo to fair it in.
