DIY With Screen Goo
You can pay thousands for a good screen, or paint your own for the low hundreds. Screen Goo Americas (probably the company with the most memorable name in the business) offers four flavors: Reference White (roughly unity gain) HIgh Contrast, Max Contrast, and Ultra Silver 3D (high gain, preserves light polarity). All of them may be rolled or sprayed on an appropriate flat, smooth surface. The even make a screen paint for rear projection! It's also said to be flexible enough that the screen can be moderately curved after painting. We're not saying that it can equal a professionally produced screen, but the demo we saw looked mighty impressive. If the cost of a screen is keeping you from acquiring a projection system, this approach might well help.