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. The bottom lens is an 85mm f/2 AiS Nikkor. The perfectly-matched lugs on Nikon rear lens caps actually "lock together"very well,and can easily be either taped or glued together. 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 is PERMANENT.
Until you have tried this method, don't knock it! So, 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 individual lens on the far right side of this photo 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" when their rear caps are attached in this manner.