holster bag question
I’ve considered this but unless the extra lens can be secured and padded I’d be afraid of it crashing against the other equipment.
Buy some 6mm craft foam, and glue a piece to the bottom of a replacement eBay rear lens cap. Cut around the cap when the glue dries, or it might be better to cut the shape to fit the inside of your bag where the lens will rest, probably a square or rectangle. That might keep the lens from moving around in the bag, and will for sure always keep the equipment separate.