Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4788303.

joints for building things with dowels.

I was going to make a shelf, then suddenly I didn't need one, so I haven't tested these extensively. There's holes in them so you can drive a nail or drop some glue in to secure the dowel, but maybe they can be scaled down to 99% for a tighter fit. They shouldn't be loose, but it doesn't take many joints for things to get wobbly. I bet not all dowels are exactly the same anyway.