I've heard of people making frames out of pvc piping and then attaching the mylar to those frames so that they can be removed and cleaned occasionally, plus it creates an airspace between the wallboard and the mylar preventing molding of the wallboard.

I don't have any REAL mylar, I just used some leftover wrapping paper I had around, its ghetto but it works better than unpainted wallboard...

Next grow, real mylar on pvc will be the plan.