Depending on circumstances, I've used Aleene's Tacky Glue, CA glue, regular old yellow wood glue, white carpenter's glue, Weldbond and probably a few more. If the fretwork is relatively thick and stable, I'll only glue it in a couple places, to hold it on the backer. If this is just a display piece, it really isn't going to be under any stress, so over gluing just causes problems, like squeeze out, that I'd rather avoid.