So i originally had one of these:
https://www.amazon.co.uk/gp/product/B07PVLJBX7/
Totally bomb proof but very heavy and over the top if you ask me. Now i just use any old silicon case.
Although the camera stopped working on my Note4 so i've been using my normal phone (Huawei P30 Pro) more recently for surveying. It's almost impossible to draw the cave up whilst in there, but we're only finding ~50m a week at the moment, so it's easy to draw up properly from memory after each trip.....