If I were gonna try this, I'd probably get a block of florist's foam. Jam it down on top of the comb to get the mating profil right. Then widdle the foam down to my desired shape for weld. Once that was done, I'd do a fiberglass build up on the weld face only to an 1/8" thickness or so. Let that cure, then repeat the fiber on the mating face side. After some light bondo filling and sanding where needed paint as desired. For mounting, I'd use 45 mil RP foam VHB tape from 3M. The extra thickness will make up for any dimensional errors on the mating faces. I don't know if you've ever used the RP series of VHB.. But it's worth tracking down a roll. It's automotive/aircraft grade, and is no joke. I've used it in several structural applications.
This'd get you to having a custom fiberglass cheek piece without doing any permanent mods to the stock. If you ever decide to take the piece off, you can soak the tape joint in PB blaster or WD-40 and it will eventually come off easy.
Or.... I've got one of these I'll sell you for $30. Don't need it anymore.