IMHO, 1/8" aluminum flat bar stock would be stiff enough for a secondary mounting point like that. 1/4" would be overkill. The lighter guage is super easy to work with; being easily cut with tin snips and easy to drill. I used hand tools to fashion some of that into fender mounting hardware on my latest
Super Course build.
Plus it's so cheap, if you botch the first attempt at making the hardware you need, you've still got plenty of material left for the next four.
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● 1971 Grandis SL ● 1972 Lambert Grand Prix frankenbike ● 1972 Raleigh Super Course fixie ● 1973 Nishiki Semi-Pro ● 1979 Motobecane Grand Jubile ●1980 Apollo "Legnano" ● 1984 Peugeot Vagabond ● 1985 Shogun Prairie Breaker ● 1986 Merckx Super Corsa ● 1987 Schwinn Tempo ● 1988 Schwinn Voyageur ● 1989 Bottechia Team ADR replica ● 1990 Cannondale ST600 ● 1993 Technium RT600 ● 1996 Kona Lava Dome ●