I put this openscad design together to allow me to mount some of our wildlife cameras on the same pole that the bird feeders hang from.

UPDATE (2019-02-10) - added the ability to change the angle of the horizontal pole, at the request of @DanRom624

The available parameters are:

clamp_height - height of the clamp part
clamp_pole_diam - vertical/clamped pole diameter
screw_hole_diam - diameter of the screw holes
screw_recess_diam - diameter of the screw head/nut recess
nut_type - number of faces the nut has (4=square head, 6=hex)
pole2_diam - the diameter of the horizontal pole
pole2_angle - the angle of the horizontal pole

More info: http://www.sodnpoo.com/posts.xml/parametric_pole_clamp.xml