Nathan, now that I think about it, you might be able to get away with using a solid rawhide tube. Just find some tough rawhide about 6" wide, soak it in water, put the two halves of the bow together, wrap the rawhide around several times to build up the thickness to about 1/2"?, add some hide glue to make the rawhide stick to itself, wrap tightly with an ace bandage, and let dry. As long as the tenons on the billets are tapered slightly, it shouldn't be too much trouble to pull the bow apart and the rawhide sleeve would be a perfect fit.