Step 4: Put them together
Because the halves are pressure fit, you'll need to poke holes in the top to let the air escape. It just so happens that we need a filler hole anyway, so use the thumbtack to create one in the center.
Now carefully align the halves so the top fits inside the lip of the bottom. You'll probably have to use a shim made of left over can pieces.
Once they are aligned, squeeze the halves together as tight as you can, making sure to keep them even.
If you're having problems getting the halves together, use the sandpaper on the inside of the bottom half to remove a little of the aluminum. It also helps to smooth the cut edges of both halves to allow them to slide together easier.