Heh heh. I love flags! Go over to the DALMAC forum - that ride is famous/imfamous for making all participants mount a 6' flag on their bikes as they ride the event. Although everyone finds a variation, the simple mount that comes with a cheap whip/flag is a simple plate that slips down between the (a, for you deltas) axle nut and the dropout.
I routinely offer a flag with the bikes I build and sell - I get them at the local Farm and Fleet store at $6 each. I do not use the axle mount, though I do use the little sheet metal plate that comes with the flag. I cutoff the tabs, drill a small hole, and use a scrounged keeper clamp from an old coaster brake bike to mount the plate onto the chainstay of my bike. Easy.
ADC
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ADC - The right bike is the one you will ride.