This is a Dovetail adapter and spacer for use with an EQ-1 telescope mount.

I printed the long dimension of the two clamp parts vertically to give it more shear strength in the clamping direction.

I used 3" #10-32 screws for the clamp and 2-1/2" #10-32 screws to attach it to the mount, although both sets could have been shorter.

On the larger clamp part, the holes for attaching the other clamp part are recessed (they were actually designed for an M6 threaded insert by the designer of the original part). If you use #10 screws, make sure you pull the head all the way down into the hole the first time you tighten it up (it was a tight fit on mine), because if you don't it will work its way down and loosen over time.

I started with this dovetail adapter: https://www.thingiverse.com/thing:2060384.
I lengthened it and added a spacer using TinkerCad.