I've worked with rayskin (same') a lot for Japanese swords. The trick is to work it when it's warm and soft - soak it in hot water until very pliable, then you can cut it with heavy scissors (I actually use tin snips). Make your pattern first, cut the soft rayskin to
the basic shape, then finish with a grinding wheel or if precision is needed, use a Dremel type sanding drum. Once you get the
hang of it, rayskin is really not difficult to work with. Also, be sure to mold it to shape before it hardens again. I use cardboard and strong rubber bands to hold it to shape. Of course on a sword handle you're trying to get a large piece to take an odd shaped, slightly cylinderical shape. Doing an insert for a knife shouldn't be anywhere near as difficult. You'll also want to sand the surface some. Rayskin has very sharp little points. I doubt you want to be gripping that on your knife.
Rich S