Taco Marine has some Extruded Aluminum shapes, but I think what you are looking for is a "Z" shape, which I don't see there. You will want to find the shape in Anodized aluminum to prevent corrosion, or you must keep it painted or powdercoated.
http://tacomarine.com/cat--Aluminum-Trim-and-Moulding--cat_aluminum_trim.htmlAnd here are anodized shapes from Eastern Metal, Marine division:
http://www.easternmetal.com/pdfs/aluminum-anodized-marine-extrusions-pipe-tube-bar.pdf
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"Knot Easy" 2000 Horizon 240 Volvo 5.7GS /SX
tow: 2017 Honda PILOT EXL-AWD
prev. boats:
'87 Chaparral 198CXL 4.3 OMC Cobra
'69 Jetstar 16ft Ski Boat, 115hp Yamaha
'68 Aluminum Jon Boat, 3hp Sears
'64 Water Wings