On the alleged superiority of acrylic vs. glass, I have a lot of 3x3" acrylic filters, mostly made by Ambico when they were trying to compete with Cokin. These come up fairly cheaply on eBay, so it's not too hard to accumulate lots of them, and there are many holders that will handle 3x3" filters, if that size is large enough for you. Voss, for instance, made a clip on 3x3" filter holder, which I use for all sorts of odd series sized lenses. I happened to acquire enough of the Ambico adapter rings (these are not so easy to find) when it was in regular production to make it worth while, though now I have several lenses that need a larger filter, so I'll have to find another system eventually.

In any case, I tested all my filters of various types once for flare, and while I found that even a cheap glass multicoated filter was better than a fancier glass single-coated or uncoated filter in supressing flare, the acrylic filters fell somewhere in between.

Acrylic filters can have other problems, such as ND and ND grads not being really neutral, and I'd be concerned that acrylic sheets not made for a specifically photographic purpose may not be optically flat.