coolant reservoir cap

posted in wanted forum, but this seems to be where the action is, so here goes again.....missing coolant reservoir cap for 1979 470/170 that i may be buying saturday(?) pending inspection of the outdrive. bugs me that i can't find cap on the net. don't really want to run with a rubber band and saran wrap.
where can i get said item? i better like the outdrive, or i'm gonna be building a deck chair with the teak i just bought rather than re-trimming a '73 v20....

thanks, hope to be posting pics of the prospective "happy hooker II" soon....