Also, whatever you use to join the front and back together, look at the joint seam. If it is very noticeable, scrape it down gently with a #11 blade, held vertical to the surface.
Sand the base flat. It may have joined at an angle which prevents full contact with the surface under it. Sanding with a fairly coarse grit (100 or 60, e.g.) also provides more "tooth" for the bond. SIG-ment works; so does the RC56-type canopy glue, and epoxy, and aliphatic, etc., etc...
If you have a prominent moustache, beard or sideburns, paint them on, too...