To solve the problem I have fashioned tethers for the lens caps out of a bicycle inner tube.
Step 1: What You Need
2) Lens caps!
3) A sharp craft/utility knife. The sharper the better. A clean cut means the rubber is less likely to tear.
5) Hole punch. I used a plain old office paper punch but you can use a leather punch or anything similar. Something that will make a smaller hole than my paper punch will probably give a better result
6) Metal straight edge or ruler
7) A coin (optional)
Step 2: End Caps
Use the clamps to straighten the tube. Don't stretch it, just straighten it. Now cut the inner tube open and unfold it.
Cut a rectangular strip out of the rubber. A 20mm width worked for me -- twice the depth of the cap. A length of 180mm worked for me. You might need to experiment if your binos are larger or smaller than mine.
The corners should be rounded. I used a coin and cut around it.
We need to cut slits in the rubber strip which will stretch around the binos and lens cap. To stop the rubber ripping we use the hole punch at each end of the slit.
Punch a hole at one end of the strip equidistant from the sides and end. We will cut a slit the same length as the diameter of the cap, so use the cap to measure where to punch another hole. Finally, using the straight edge, cut a slit between the two holes.
Now do the same at the other end. This time the length should be the diameter of where the tether sits on the binos.
Carefully stretch the rubber over the end of the binoculars and the end cap.
Step 3: Eyepiece Cap
I guessed how large to make the slit for the cap and stretched it out sideways when fitting.
At the other end I cut a slit right to the end. This wraps around the centre shaft. I wrapped it with insulation tape to attach it there firmly.
Step 4: Final Notes
They are easy to use, simply pop off and don't worry about losing them!
I hope someone finds this instructable useful!