Storage shelving coupler adapter

This mimics the injection moulded inserts, that ship with [Raybee metal storage shelves](https://www.amazon.com/dp/B098X5TWKF/).
Intended to go between the bottom and top leg levels, instead of using a shelf as a coupler.
Includes 10mm square bed adhesion pads, that need to be cut away with a utility knife after printing.