Here is another use for Tool Dip, the clear kind. Keep those instruments safe and clean!
Take your two instruments, clean with acetone, add velcro or double sided tape to each backside and stick em together. I recently began to color the two sides with magic marker so its easier to know which side is which in cave. I run a small bead of silicone over the seam (I used to use electrical tape but the solvent in the Tool Dip made it gummy), attach a bit of fishing line for holding to the two brass rings and put a small strip of duct tape over the two lenses.
Dip in the Tool Dip.
If the Tool Dip is too thick the proper thinner to use is xylene which also helps in keeping bubbles down which are of course much easier to see since its clear. It will look all bumpy and snotlike but as it dries it pulls itself together and makes a nice smooth jacket of plastic around your instruments. Cut around the duct tape over the lenses and remove. I then cut off the fishing line and clean up around the brass rings so they move freely and silicone any gaps and attach my cord of choice.
I have had numerous people use these in a survey and no one has had any problems with visibility or anything. If needed it peels off very nicely and leaves no residue behind.