Wire Shelf Clip-On Hanger (Prints Flat)

They snap on to the front or side of the shelf. Sized just right to be springy on shelves… a few brands have smaller rails, so you might print it at 95% or so if you need a tight fit on those. 

Since these are flat, you can stack them to print a bunch in a column. Putting them .25-.35 above each other, depending on your layer size, works fine for me. I fill in empty corners when I'm printing something cumbersome :)

I recommend PETG or a strong plastic with a few solid layers. These can technically hold a lot more eight that you're likely to put on them due to the geometry (like, 100lbs would not be out of the question I think, but I don't recommend that much). I'll post some photos of them in use, and maybe an informal tensile test. I recommend keeping it to a weight you're comfortable with knocking off the shelf, just in case.