VERY MUCH IN DEVELOPMENT AND USE AT YOUR OWN RISK! DO NOT HANG YOUR EXPENSIVE LENSES FROM IT SHOULD YOU TRY TO MAKE IT! DO NOT BLAME ME IF YOU BUST A LENS!

A little more complicated build as you will need to 3D print some parts, buy in some bolts and screws and do a bit of metal work to produce the mounting plates and belt buckle.

This device is hooked on your belt. The idea is there is always one lens mounted on it, a second on your camera. So, to change lens, remove from your camera and clip into the open spot on the device, then remove the other lens and put on camera. This way you don't have to put a lens down.
VERY IMPORTANT: If you decide to make this the bolt must have a LOCKNUT. The pins must be made from metal as must the top and bottom mounting plates and the BELT BUCKLE. I am going to fashion the pins from stainless steel nails I have and manually cut the mounting plates from Aluminium. The springs are just generic ones I have lying around but it is important that it has a smaller diameter than the pin head. There is an image showing where the pins and springs goes.

DO NOT 3D PRINT THE BOLT, NUT, SCREWS, SPRINGS, PINS, BELT BUCKLE and MOUNTING PLATES as I have included those only to give an overview of how the devcie comes together and they must all be made from metal. The mounting plates should be used as a paper template only. They are a Nikon F-Mount but may need a bit of fine tuning.

The bolt is based on a McMaster-Carr part but I found Zenith brand stainless steel bolt M5 x 20mm and M5 locknut locally. The McMaster parts are 90576A104 and 92005A322.
The 10 screws, also based on McMater-Carr ( 90065A151) but again I found a Zenith 6G x 18mm stainless steel part locally.

Note at this point the holes sizes and counter sunk dimensions for the 10 screws in the mounting plates are wrong and I have not bothered to fix it as I will eyeball it when I make them as you want to ensure the screw heads can't scratch your lens mounts so the counter sunk will be adjusted accordingly.

There is a commercial device like this available but I do not own one, nor is this directly copied from it. In fact, I've never seen one in the flesh. If I am breaching copyright, please let me know and I'll remove it. It's just that I have wanted one from the first time I saw it on the web so I thought I'd give it a go myself.