There are a few styles of canopies, please be a bit more specific on what type you'd like. Additionally, what type of lights will be housed inside of said canopy?
Once those items are narrowed down it will be easier to direct you to diagrams etc. However a simple search on the forums and through the net resulted in a few helpful links.
Personally if you're going with a high heat lamp (halide or equiv) I'd make sure you have vent fans to help keep the tank stable.
Another consideration is how easy it will be to change lamps in the fixtures. I like some of the new top lifting canopies I've seen lately because there is no need to remove the fixtures and it provides very easy access to the furthest back lamps.
For a basic canopy design it's just a big box with a skin applied and some trim. I use brass piano hinge when I've made or designed canopies as it tends to last much longer than the silver hinge. You can use locking hinges as well but once again I'd stress using brass for the hinge pieces as it will last the longest.