Everything about this model is designed with 3D printing and it's limitations in mind. Make sure to have the top layer surface as aligned rectilinear, with the lines in the direction you see in the photo. This will help make the inside surface even smoother to decrease friction. I printed mine out of PETG.
I designed this one for 6mm connectors, since that's what I had lying around. If anyone needs one like this for the 10mm connectors, let me know and I'll make another version!