I made one. Unfortunately it's on loan or I'd photograph it, as a starting point for you.
It is basically a rectangle of 4" heavy RHS with two sets of 3 pt mounts on one side: holes to pin to, and pins. They are spaced about 5 inches vertically to give clearance...and lugs to take pins for my FEL. I made it so it can be used either way around, eg I can put 3pt attachments on the loader or loader attachments on the 3pt. Handy thing when you need a handy thing. I put a spray boom on the FEL with it etc.
Edit: I made a stand for it that pins on where the top link goes, to stand it up and make it easier to hook to the tractor. Then that pins on the side when not in use.