The area my nest boxes are in has no trees, utility poles, or other existing objects to mount on. Also, the ground is very rocky which makes digging a hole to install a post difficult. My solution was to use 2 3/8"tube steel fence posts and matching brackets (called "pipe grip ties" or "wood to wood rail ties") to attach the boxes. Short screws were used so no sharp points extended inside the box.
Steel T-posts were driven into the ground, or existing ones in a fence were used, and the tube post slid over the T-post. A short section of wood was driven into the top of the tube posts to prevent foot injuries to the birds.