When you detect, the coil gets dirty due to collisions with the ground.
It becomes more and more heavy, therefore it must be protected.
This is my first solution for protection of the coil:
The protection is made from a bathing cap (bonnet de bain/ Badekappe).
After detecting, it is very easy to clean the coil: just wait that the dirt has dried, then it falls down.
After 3 monthes of test with the bathing cap, I was slightly satisfied but the bathing cap need to be replaced due to damages of chocs with stones.
Therefore I found this second solution with a more robust material: it is a bag for storing shoes of golfers!
For 3 euros at local shop "Decathlon", this material is much more resistant than previous and after one month intensive test, there seems to be absolutely no damages.
The only thing I had to take care, was to fill a hole in the middle of the circle with some thermal glue (see red arrow).
There was also a small metallic ring at this place that I removed otherwise the detector would have detected it permanently.
Additional weight: 30 to 50g.
It seems that this bag is not easy to find at the Decathlon-shop, therefore please find hereafter the scanned reference: