Schaller Oyster D/S Piezo

I am using them inside a my Fender Concert Tone FM53S mandolin.
The sound is great, but they are too heavy and the glue is not strong enough to hold them inside the instrument. that is why they are falling all the time and i tried many was and glues to keep them in.
In the end I changed them to the K&K mandolin twin spots that are sound phenomenon.
I tried them also on a normal acoustic western guitar and they are sound ok. I still prefer the K&K ones LR Baggs and even the Duncan ones! I will not bu them again.