SKYPP I worried about that as well. If the plexi your using is long or thin I suggest you drill 2 very small holes in the center and feed a piece of fishing line through. You can then attach the line to the ceiling(adjust as needed) giving your plexi center support. If you use small holes and thin line you shouldn't harm the effect any.
I'm not exactly sure what the measurements are for my piece of Plexi, maybe 1 1/2 x 3?. I bought it at LOWES for around $10. I'll get back to you on a later post. Sorry I'm at work right now.