This Riflescope 2032 Battery Holder from Vortex is a rail-mounted spare-battery holder. It is designed to hold a CR2032 battery, and it will attach to most Weaver or Picatinny rails. The attached security lanyard ensures that the battery can be retrieved, even if the holder slips off the rail for some reason.
So simple, so easy... why is this the only product of it's type? Bought one years ago for a 1-4x Vortex something or other now discontinued but still going strong on it's third host. Every rifle since (whether using a Vortex or not) has gotten one if the optic uses a 2032 or 1632. Saved my bacon on more than a few occasions. At this point it's just a given. The tethered cap is a fantastic touch.
Came across this after my battery drained down on my Viper PST while out on a hunt. . . . Never thought to carry a spare before and started looking at options after this happened. I can fit 2 batteries in there with room to spare and never have to worry about being without an illuminated recticle again. The small/sleek profile makes it barely even noticeable on my 110BA .308.