Ok, here are a couple of very quick renderings. I'm not sure what color you want, so I used black, which doesn't really show up that well in the pics.

The inside would actually be white, I just didn't do it for these.
The way I designed this is, the top is 8" above the tank, and the sides extend down about 1" to cover the plastic tank trim, and to provide a lip to lock it in place. So visibly you see 9" of wood.
I put the hinge right in the middle of the top, which should work out well. If I can get the right kind of piano hinge, you should be able to flip the whole thing back so that it can rest on the back half of the hood.
I haven't put a price together yet, but I don't imagine it will be very much, especially if it is going to be painted because I could use all pine. I'm thinking in the $50-$75 range, although I haven't priced piano hinges either, so I don't have a feel for the cost of those. I will try to get the pricing together soon, as long as this concept is what you were thinking.
Picture with hood closed:
Picture with hood open:
Let me know what you think!