thank you all for the information and encourages that made this possible! a quick report with some photos, hopefully this provides some hints to the fellow rvers in case anyone attempts to do it as well...
(1) got a 3 inches x 2 inches x 10 ft roof-to-wall galvanized flushing sheet metal, for $6.57 at home depot.
(2) calculate the length. in addition to the length of panel (dam length), add 3 inches to each end to tuck backward. my total length was 61.5 + 3 + 3 = 67.5 inches.
(3) the leftover section was to be glued to the dam so to make it more rigid.
(4) considered the panel brackets and cut out that areas on the sheet metals.
(5) glued the two pieces together with "fuze it" ($6.97 at hd). let sit overnight
(6) attached the dam to panel frame. i planted a 5/16" galvanized screw to the middle of the front panel frame, and cut a slide channel on the dam back side. when installed, a support at the middle was in place in addition to that at two ends.
(7) affixed two ends to the panel siding with 3/16" rivets (toward left is the front)
(8) i wrapped the the front dam center edge with a wiper blade protector and dicored the edge where it touches roofing. notice the curved roof, only the middle section of the dam touches roof.
now it is completed. hopefully it will provide a better aerodynamics.