I worked on the frame again tonight.
First I popped some holes in the side pieces. Yep, the excess tab still needs to be cut off.
Then I popped some matching holes in the top and bottom pieces. I also countersunk them so the insert should slide right into the hole that is cut for it. At this point I was glad I was using 1/8" thick aluminum vs 1/16".
Then all that was left was fitting and putting them together. In order to get things to line up I had to do a little slotting work with the drill and a bit of elbow grease. Thankfully its only aluminum so it eats through it pretty easily.
Here is the frame assembled. As you can see my camera kinda died on me at the end, but at least it got the whole frame in the image. It looks pretty good. Its not perfectly square, but I'll work that out another time by messing with the bent tabs.