I looked at the web page mentioned. As far as I can tell the lips on the edge of the channel are for appearance. A similar box, that would be just a strong, could be made with standard 3" aluminum channel. Just make the covers a little larger.

BTW: Did you think of asking the owner of the website mentioned, "Where he got the aluminum frame extrusions?" He has a mail me link on the main page.

Edit: OOps, just noticed you had written guy