Mechanical fasteners and epoxy are the best method. Corby bolts and epoxy are the most secure.
If using only adhesive, surface preparation is essential. Rough up both surfaces at 100 grit, clean with alcohol, use slow cure (24 hour) structural grade epoxy. Don't clamp hard. Use just enough pressure to hold the scales in place. Too much pressure will squeeze out all the resin. There care many brands, but the most common are West System and System Three. Many knife supply companies carry a epoxy made especially for handles.
Another excellent method of attaching scales with no pins or mechanical fasteners is using the 3-M VHB tapes. These come in thinness for .001" to .030". I found that the .005 to .015 worked well. They mount marble and granite slabs to walls with these tapes. Once allowed to fully cure for a few days to a week, the bond is super strong.