This is an extension tube I made for the Goodman Art Adapter to connect my EF and EF-S lenses.
The AA's build guide demonstrated assembly with only a Nikon extension tube, so getting the right tubes for a Canon would be difficult for the layperson. My extension tubes were too big to fit properly on the AA and I didn't feel like sacrificing them. Therefore I decided to make my own.
The mount is based on the rear lens cap of an EF lens. The lens is held in place by friction only since making a mechanical button to hold the lens is beyond my skill.
The tube itself does not have holes for M3 screws to pass through. Rather you will need to drill out the holes by eye. You can use the AA's tripod mount as reference for drilling the holes.
I uploaded the STL file here for anyone to download.