Scott Kennon said:
What, have you tried this before or something? I wouldn't "have it coated" anyway. That's the point of this... DIY.
You asked a question, myself and others spent a good deal of effort to explain why a piece of Plexiglas would not be a good choice to use for making a telescope mirror. I think it's only fair if you ask a question that you take the time to try to understand what has been said. I believe that if you took the time to understand the accuracy required to make a telescope mirror, the properties needed so that one can actually make a mirror for a telescope, then you would understand why it would be a futile effort.
The goal is not to DIY, the goal is to do things that are reasonable and make sense. As an materials engineer, I could show you a variety of calculations that would demonstrate the various difficulties with this particular material but instead, I tried to explain in terms that were more understandable.