This design uses up to two M3 threaded bolts to secure to the tree. I sized it for my tree with a 0.29 inch ID collar.
Prints cleanly with organic supports, you really just need to support the two sections of flats at the bottom of the part and to the top. I've chamfered these faces to reduce the low angle overhangs. See photos for my print setup.
I've included the .SLDPRT if you have access to Solidworks, and if so feel free to remix and redesign to your heart's content.
The author marked this model as their own original creation.