Back in the '80's I started molding custom holding fixtures for each knife I engraved using wooden blocks and a two-part epoxy putty. I believe it was Tom Overeynder who came up with the idea. I made many this way and eventually replaced the putty with Bondo because it was faster, easier, and cheaper than the epoxy putty. Now I use Thermo-Loc.
The fixtures I make are molded to each side of the knife and fit the contour perfectly, so stresses are distributed evenly as it's tightened. To this day I believe this is the best type of system for holding custom knives. As for the GRS fixture, I tested early models made by Bob Finlay (the inventor), and while it works as designed, I prefer a molded fixture.
Ron Smith's right...these high-end knives can be very touchy. Even Hoels and Busfields with their superduper tapered pin arrangment can tighten a bit, especially when doing gold inlay. I used to shim the insides with micarta or wood cut to fit, but abandoned that because I wasn't seeing a benefit.
Beginner knife engravers should take whatever time necessary to make a proper fixture for knives. If not, it can cost you dearly. Many knifemakers are skittish about working with a new engraver who may not have the experience in both holding AND refinishing their knives after engraving. Some prefer to have the engraver work directly with the customer so they're out of the loop and assume no liability. I've spent many hours talking to makers about holding their knives and how they achieve their final finish. I've gone as far as to get private refinishing lessons in the hotel at a knife show. These guys are masters at what they do, and there's a LOT of good information to be learned from them. Communicate with them! It shows you're serious about doing a good job on their knives, and they appreciate and respect that. It also makes them much more comfortable dealing with (and recommending) you. / Sam