Community Book

The box doesn't have to be completely enclosed, but just mostly enclosed. (Mine has the projector on a drawer that slides out to provide enough throw distance)

Why so little attention paid to the surface? You need a thick enough surface to not flex when pressed (to avoid short-term hotspots becoming blobs) and some kind of surface for projection and diffusion. People have used vellum, tracing paper, fabric interfacing, and even cheap plastic tablecloths or shower curtains (in my case, what worked best) on top of the surface, in addition to frosted glass or acrylic. Before purchasing a frosted material and/or diffuser (or before return periods run out, if available) it's wise to test the surface's suitability both as a projection surface (clear image, doesn't let you see the bright projector - this problem hit me) and as an IR diffuser (doesn't block too much IR, produces nice contrast)

Mirrors: many people suggest front-surface mirrors to prevent ghosting/double images, though for testing purposes or less demanding uses, normal, cheap mirrors work fine.