Now that the legs are folded out, put the main stand away and get out the cross-bar again.
You will need to cut out the inset slits so that the crossbar will fit over the main body legs. Make the width of the slits according to the thickness of the pizza box (or matte board) that you chose to use.
I used a scrap of cardboard leftover from the pattern to test against. Just gradually widen the slit until you get a nice snug fit. Be careful that you don't go to wide or things might get a bit wobbly and loose.