I think I got the same thing as Smudge. Here's a pick:
It's a bit bigger than 1/4", closer to 3/8" but I've not had anything make it through, but the operative word might be "yet"
I went to Home Depot, in their screen door section you can get the rails, corners etc to make the frame.
When I made the one for my 77, I made little "U" shape cutouts to go around my returns which are over the rim. Don't do it, it sucked, looked horrible and really made the back of the frame weak so it tended to sag. I even used corners and little cut rails to go around everything.
If I were you, I'd make it as wide as I could from front to whatever sticks out the most in the back. That will leave a bit exposed probably between the frame and the actual rim at the back.
I also tried egg crate, but I found it probably reduced the light in my tank 20-30% (rough guess).
Hope that helps