I keep 3M-VHB in several thicknesses. It is amazingly strong and according to the paperwork, will not break down with age. The bond actually gets stronger for the first several months. Ebay is an excellent source for it, as are industrial adhesive suppliers.
This seller has it in nearly every bond grade and thickness.
http://www.ebay.com/itm/3M-VHB-9473...736?pt=LH_DefaultDomain_0&hash=item19d62d59d0
Tips:
1) Everything has to be a dead flat fit. These sheets have adhesive from a few thousands to .100", so pick the right thickness for the job, and assure that the two surfaces meet perfectly.
2)Don't use the first 1/8" around the edges of the sheet. It may be less sticky due to the minute exposure to the air from the side.
3)Trim the sheet to a little oversize for the scale, remove one paper backing and place your scales in the center of the sheet. Rub gently from the paper side, then trim around with a fresh scalpel/#11 blade. The scale will have a perfectly fitted adhesive backing. Peel the second paper back and carefully place the scale on the knife tang. Get it right, as there is no moving it easily. Place it very gently and check the edges to see all is right ( not needed with an inlay) if all looks good, press down to assure a good bond. Rub all over with very firm pressure ( cotton gloves help prevent sore fingertips) for a while to make sure all the adhesive gets contact.
4) Set aside for a day or two to bond up strong.
5) If there will be grinding and shaping of the scales, use gentle pressure and avoid heat.