Peerless also offers projector enclosures similar to the Chief model and I'm sure there are other manufacturers that have similar products.
Making your own enclosure may be possible but there are a number of things you need to consider such as how the enclosure attaches to the projector mount or building structure (and is the mount rated for the additional weight), the ability to protect the projector from all sides, provisions for proper airflow and ventilation, easy access to change lamps and clean filters, fire and smoke ratings of the materials (it is near a high heat source and will be in an occupied space) and so on.
It's probably not feasible here, but in similar situations, and when the budget supports it, I often look at using projector lifts with a plenum enclosure. You can mount the lift up in the trusses where it is much more out of the way. With a bottom panel and plenum enclosure the projector is essentially in an enclosed box when not in use and then is lowered down via remote control for use. With a 3 position lift with a long drop you can even add a service position that has the projector several feet off the floor for easier service and maintenance. Not a cheap approach, but often well worth it.