On the left is the silver-colored 45-P Nikkor with its special grey colored rear lens cap fitted to a regular Nikon rear lens cap,and simply taped into place using blue masking tape so you can easily see how the perfectly-matched lugs on the caps actually "lock together" these reverse-mated rear lens caps. The connection between the two caps,with probably over 100 small interlocking lugs, is a VERY strong connection,even with just a couple of wraps of gaffer or masking tape; with epoxy added the bond would be completely PERMANENT. Until you have tried this method, don't knock it! On the left we have the 45-P with its NC clear filter and special "bowl lens hood" fitted,and the Nikon BR2 lens reversing ring mounted on top of the NC filter. The BR2 does not interfere with shooting or anything,and just rides along there with the 45-P,ready for use. The bottom left lens is the 85mm f/2 in AiS mount, and the right lens is the 35mm f/2 AF-D Nikkor for a size comparison. In actual carry, two small lenses like the 85/2 and 45-P act and carrry almost like "one lens".
In smaller,compartmented camera bags, stacking two short lenses allows you to securely and safely "fill up the space" with securely-fitting caps which will not allow lenses to get scratched. The REAR elements of lenses are the most easily-harmed part of Nikkor lenses; a scratch on a lens's front element is typically no big deal; the same size of scracth on a lens's rear element can spell disaster; by using this method, you always keep a rear lens cap on the lenses left in the camera bag. In a very small,tight storage space like a belt-worn lens pouch,or any type of tubular case,or in a drawstring pouch, using two rear lens caps back-to-back allows you to carry two lenses, or a lens and a converter, in a safe and secure fashion instead of having two items which could bounce off of one another or compete for space. Using back-to-back rear caps will allow you to carry that extra item in space that might otherwise be wasted.