Sounds like you have a plan that would work. I would just build two boxes without a face with dimensions that would allow one side to sit against truss so you can screw the truss and box together and cover speaker obviously. Bead of caulk on perimeter just before setting box in place.
A full enclosure would be better so you could mount the speaker onto a better and more stationary surface than drywall. The best speakers have cabinet fronts that offer no flex when driver is moving.