Bluebird houses (nesting boxes) require specific dimensions and design features to attract bluebirds and protect their nests. Some of these features include a specific size of access hole, and internal “ladder”, and a “no perch” requirement. Here is a useful link on bluebirds and their nest boxes :
https://www.audubon.org/news/how-build-bluebird-nest-box
This design is based on the guidelines in the above link, with some minor differences to allow for 3D printing. Print in PETG.
 

Note that this print requires additional hardware; typically :
MountingBlock :
M4x16 SHCS with washers and hex nuts (qty 4 each)
M6x35 hex head bolt (to mount to post)
Thumbscrew and Lock :
M4x30 SHCS and hex nut
There are five parts to the nesting box, plus one additional part for mounting to a pole (choose one of 3 options or screw directly to tree).

Assembly:
Additional parts required (stainless steel hardware is recommended):
M4x16 SHCS with washers and hex nuts (qty 4 each)
Thumb screw - M4x20 SHCS and hex nut (qty 1 each)
M6x35 hex head machine screw and nut for large post mount
Note : The post mount hardware needed depends of course upon the type of post you use. I used a 7ft (2133mm) U-Post.
 

Begin the assembly by adding the four hex nuts to the hexagonal recesses in the MountBlock - four on the back side and two on the front side. I recommend using a SHCS and washer inserted into the hex nut through the hole opposite the hexagonal recess, and then tightening the screw to pull the hex nut into the socket (the hex nuts will be a tight fit). Be sure to insert the M6 hex head mounting bolt into the back of the MountBlock before attaching to the Body.
With the hex nuts and M6 bolt in place, fasten the MountBlock to the back of the Body using SHCS screws (with washers). 
Next, slide the Roof piece onto the body - back to front - as far as it will go. Do not force it past the stops.
Prepare the Lock piece by inserting a M4 hex nut into the hexagonal recess using the same method as before. Slide the Lock into the dovetail recess in the Bottom. Note that the Lock piece has “crush ribs” on the dovetail section, in order to make it a tight fit into the dovetail slot.
Slide the Bottom piece onto the dovetails on the bottom of the Body from the back; line up the the slot opening on the Bottom with the slot opening on the Body.
Finally, slide the Front piece up into the grooves in the body, being sure to push it all the way up into the recess in the underside of the Roof. Note that the Front piece has “grip slots” on both the front and back sides - the long slots should be on the inside. Secure the Front in place with an M4 thumbscrew.
 

Thanks to Kaje @kaje for this model.