The only thing that I don't like about rubbermaids is that they tend to bow out a little when filled with water. So when you buy one you want to maek sure you compensate for the bowing. Other than that I think they are the most economical way of setting up a fuge.
I guess it depends on how much room you have. Bigger is better. If you have room under the tank you can always convert a smaller aquarium as I did for my 25gal tank. You could always have an acrylic built for you to fit your application perfectly. I had my sump/fuge for my 150gal built by Brent at http://www.barraquatic.com He does an excellent job is has good prices. (He is located in Spokane) My 75 gal sump made from 3/8" acrylic was only $389. Check my gallery there is a pic of it in there..... Good luck..
As for the Custom size under the tank.. since it wont be seen how hard is doing custom acrylic work Or just getting glass cut to size and making the sump myself.. Not that I have anything against Brent.. just trying to do more stuff on my own.. In the end I'm sure I'll be calling him and have it done...
Curious if anyone had tried making their own tank?