Made the bird house from cedar fence planks. The wood is not expensive. But, the wood is rough and each plank is variable in quality.
The sides are 12 inches high, with a 21 degree angle cust, bringing the other side down to about 10 inches.
Along the top end of the side, several holes are drilled to allow for air flow.
The back is about 14-16 inches, allowing for the brid house to be easily mounted.
The bottom has the corners cut out for drainage.
The whole is cut about 7-9 inchees from then bottom. It is about 1.5 inches in diameter.
I added an extra strip to the whole, giving extra depth. The ideas is that this makes it harder for preditors to get access. Other projects add a metal guard around the hole.