I can tell you from experience that it's difficult to build a 6ft canopy. It can be done but if your going to do it so it turns out nice and perfect you will need to make some special tools. Or spend a lot of money to buy some. The main "tool" I'm talking about is if your going to glue your corners together (which I recommend) then you will have to find a way to clap the whole thing together. I know your thinking you will just use 2 3ft bar clamps, but you have to remember your clamping on the outside and it's to long for these clamps.
As far as building a canopy, I like to use 3/4 inch oak plywood. Use it for the front, sides, top, and back. Make it so it fits over your tanks top plastic trim. Then you can attach strips of wood on the inside of it that will sit right on top of your tanks trim. I suggest you do a search on canopys here and see if you get all the info you need. There have been a lot of these posts lately. Here is a pic of a canopy I made for a friends 6ft 180.