The idea of a personal outdoor movie theater is, flat out, great. It's not only fun but relatively simple and inexpensive.
Sure, it could be quite expensive but, really, you're not looking for award winning sound and video in an outdoor setting. The experience itself is what's thrilling. It's not unlike a drive-in movie theater. The sound comes from your car speakers and the screen is stained, but hey you're watching a movie in your car.
This is the only time I'd recommend a power point type (read cheap) projector for home use. It's also the only time I'd say you could uyse a bedsheet as a screen. There's other ways to go as far as screens are concerned. And there's folks who are going to spend some good money, well, because they can and there's nothing wrong with "better".
Here's three outstanding outdoor theaters from Electronic House, Take It Outside! They can give you some inspiration.
If you have a question or if I may be of service email me at radiohannibal@gmail.com
John Hannibal