No, you can't build your own projector mount.
I know that guy in your church is willing and skilled to build a projector mount system for you. He will donate his time and can buy all the materials for less than $50. I firmly believe he will construct it well and assume that he will promise that there is no way it will fall. He will demonstrate this fact by hanging from it and provide stacks of documentation proving the weight rating.
However, you can't (or at least shouldn’t) let him do it. Most liability insurance policies require that a projector hanging from the ceiling be mounted using a UL Rated and Manufacturer Approved mounting system (I personally like www.ChiefMFG.com products the best).
In addition to its being a “have to insurance requirement,” the professional mounts have a ton of great features that allow for roll, pitch, and yaw adjustments to make the installation easy and the image perfect.