I hate to start a whole new thread, Circa, but, IMO, if you are going to make such a large investment, I would decide if I wanted acrylic or glass based on the benefits of each. They are arguably quite different.
Acrylic is MUCH lighter (~25%?) of glass, is 100% clear (tempered glass has a green tint), is a much better insul
ator (less temp. fluctuation), is seamless (i.e. - leakless, and quality manufacturers guarantee for life)but scratches much, much more easily than glass. Scracthes can be removed from the outside of the acrylic quite easily but it is much harder to remove from the inside (if you are careful you should not scratch it there). Acrylic can be formed to just about any shape, too.
Personally, I couldn't imagine choosing glass (I've lived with both). BUT, if you listen carefully, you may find just as many people dedicated to glass.
IMO, 250 watts is good enough for clams.
As far as the supports go, I'm not very familiar with the setup. Personally, I would look to support them better (wood?) before I add insulation.
Good luck!
McReef