Chickadee Nest Box

I have been building nest boxes for various birds this spring and thought it would be cool to 3d print one. I found a proven design that was based on 3 inch (75mm) PVC pipe which was intended for chickadees and nuthatches. Here's a link to that design: http://web2.uwindsor.ca/courses/biology/dmennill/nestbox.html
This design is intended to be packed with wood shavings/sawdust up to the hole, the birds will excavate the shavings to create the nest. This arrangement mimics a standing dead birch tree, which is apparently a preferred nest location for these cavity nesting birds.

I took the basic dimensions and made a 3d model with all the rough gripping surfaces, ventilation and drain holes built in. The box is divided into sections which thread together. The main section has a mounting bracket built on which can be screwed onto a tree, or strapped in place. The lower sections can be unthreaded each season to replace the sawdust/wood shavings.

I included a slight taper on t