Marck,
It is marine cap rail, predrilled 6" centers. Comes in 12" sticks. Very mallable 1 1/2" x 1/2" The 1/2" side has a radius to it for the body wrap trim (roof and siderwall). I found it at a local Marine Aluminum Distributor in my area. The rest is Home Depot stock stuff 3/4"channel, 1 1/2" flat stock and 1/2" angle