I've been drawing up plans for a viv for my big girls over the last few days. For me there are two reasons driving me to build my own, firstly is the cost (although the cost saving over buying one when it's in a sale isn't huge), but the main driver is that I can design in features that I want and you don't get on commercial vivs.
When I build up my 1200mm x 533mm x 600mm box I'll post some pics and the plans.

I'm intending to use acrylic for the doors on the basis that it's cheaper, lighter and easier to work with than toughened glass.