Remove the old glue residue - light scraping should work. Try not to damage the visible part of the burl insert.
Reglue with something like 3M 2216 epoxy. 2216 does not set as hard and glass-like as most other epoxies. Be sure to clean all new epoxy out of the reel seat pocket.
Drill and pin the butt cap with .040" Nickle Silver wire - two pins at 90 degrees either side of the pocket.
I have the stuff you need and plenty of time on my hands - PM me if you want some help with this simple fix.
Regards
Brent