Do you mean a lip made of silicon rubber?
I mean place a sheet of silicone over the top instead of the Velcro, and attach it to the aluminium. Cut a hole in it (with rounded corners) that is just smaller than your phone. As silicone is flexible, your phone should be able to fit through the hole and be clamped between the silicone on top and the aluminium underneath. (The silicone should also prevent the phone from sliding around due to the friction between it and the phone.)
It should then be (a bit) like putting you phone inside one of those flexible silicon cases from the top/front. - I would not anticipate you having access to any controls that were accessible on the sides of the phone if you do this, but the touch screen should be fine.
(hope pic helps with concept)