If you are going to get a screen, I'd suggest a high gain screen if possible. That will help with ambient light, and provide some extra brightness. Some mini-screens come in high gain surfaces. One example is the Da-Lite Presenter screen shown here. It is 40" diagonal, sells for around $199, and has a particularly bright surface.
For a lot less money (around $50), Draper makes a 25" diagonal tabletop screen for pico projectors. I can't say that I've actually seen one, but here's an image I found.
I'll be at Infocomm in June, and look around to see if I can identify more suitable portable screens.