Not sure how many of you Astro educators have a similar situation, but if you do, this may help. I have been doing this for several years now, while waiting for a planned renovation that will/wont take place. I like the way it works, and the kids respond well to it. It's not perfect, but it's cheap, and effective.
Step 1: Be sure you have a digital projector. I have used 3 different ones over the past 4 years, and noticed that they're getting better and cheaper. The one you have may work pretty well. then pick up a 24" quarter round security mirror. Will cost around $30. Mount that behind the mirror
You will want to set it up for rear projection
You may have to play around with the distance between the mirror, and the projector
Play around with the adjustments until you're happy with the image