Dirt, dust, mud, and water can be mostly blocked by placing packing tape over the sights on cave survey instruments. The tape in the picture has been on that instrument for several years. It holds up well. Make sure you adjust the sights before you tape them shut. I also put a piece over the capsules, for an extra layer of protection. No more licking the lenses clean (clensilinigus?).
Packing tape also works on laser rangefinders. Testing on my Bosch 165 showed no ill effects from the tape. This pic shows tape over the sensor and laser, but I have since been covering the whole thing with tape. Makes clean up much easier.