I am concerned that you lost the copper loops too... If not there are several products that could seal the space.
BCD
You might ask the Mfg or an aftermarket coil company like NEL about materials to use to fix it.
I'd say you got some serious detecting use out of that thing!
If you are getting erratic operation the coil winding is likely compromised. The epoxy secures it and seals it but just loss of epoxy should not make it dysfunctional.
I believe the erratic operation is due to the moisture getting inside where the epoxy should be. When its dry, its runs just fine. I can remember getting erratic signals for months before the epoxy came out, just wrote it off as EMI, but now I know it was because moisture worked its way into the cracks in the epoxy...
My buddy makes coils from scratch, I txt him to see what epoxy is good. Being thanksgiving he may take a while to respond but I will post it.
Make sure the coil is totally dry. Brush off loose dirt. Wipe it down with 91% isopropyl alcohol (readily available at drug store). Get the 91%. Use a scotch brite pad to roughen the surface a little. Be careful not to abrade any copper coil wire if you see it. Vacuum the dust off. Now, before you actually epoxy. Test it carefully. If your sure it works, get a good 24 hour epoxy, Devcon for instance (home depot). Make absolutely sure there is no metal in the epoxy you get. Many have metal as a reinforcement material. In looking at your issue, I'd put a strip of painters tape around the outside and inside edge as a dam. Pour the epoxy in and let it level. Once you've replaced the "missing material" let it cure thoroughly, remove the tape. I'd then go back and give the whole thing another coat with a brush. You might even contact the manufacturer and find out what epoxy they use. Epoxy and plastics are not the best of friends. It'll work, for a long time, but over time the bond will deteriorate.
Nice work and congrats! I'm curious what the temp was on that hunt? Never went above 26 here but the wind died down so I went swingin anyway!
Nice work. I had a similar problem with my 8.5x11 and a similar fix. Still working 2 years later.