I had my glass shop make a 1/8" glass top for my CADlights. Cost me $36 for two, one for a 12g and one for a 5g. I simply traced the top and had them cut to shape. Rather than having an indented / cut-out back like the picture above, mine was a simple straight cut. I left about 1.5" off the back so I could add a plastic extension like you see on the back of any tank lid - that way I could cut smaller holes for the pump, heater, etc. wires.
QUOTE (Bonsai @ Sep 27 2009, 07:52 PM)

You could always trace a template and take it to your local glass shop. I've had Plexiglass and 1/4" Plate Glass lids made with curves:
Glass:

Plexiglass with Hinged Feeding Door:

How do you rest the top on the tank? Does it simply overlap so it does not fall in?
What do you do about water condensation on the top? Doesn't it spill down the sides of the tank if you simply rest it on top?