There is a product made specifically for that application. We use them but I can't recall the manufacturer. Google "trailcam t-post mount. We've purchased ours at Los Cazadores in Pearsall. You could call and ask them.
I just put the post in upside down and secure my cams to the fins of the post. It's worked really well for me. You may need to put a small twig or something behind the edges to make up for the bumps in the post but it works. As long as you don't have livestock messing with it like said above.