I'm sorry, I thought I answered this already, but apparently not.
Layout the shape of the top you are making on a sheet of plywood. (I used the canopy) Then screw in 2-3" construction screws along the line of the layout. The first spot should have 2 screws spaced apart the width of the metal you are using +1/8" (that's to hold it into place) Place the metal into that slot you just made then start heating the first 6-10" or so of the metal with a heat gun set on high and gentle start pulling the metal towards the screws along the outline. Keep doing this until you get to the end. Do this slowly or you will bend the metal (it is thin aluminum). Once you have finished leave the piece in the jig for a couple of hours to let it settle into the new shape.