For rear dynamo light mounting on racks I used about 1/16" or so aluminum plate. I have quite a bit of it in various thicknesses that I have picked up at scrap yards over time for projects. I have made mine large enough so I can run a Planet Bike Superflash battery light as well as the dynamo taillight.
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Gear Hubs Owned: Rohloff disc brake, SRAM iM9 disc brake, SRAM P5 freewheel, Sachs Torpedo 3 speed freewheel, NuVinci CVT, Shimano Alfine SG S-501, Sturmey Archer S5-2 Alloy. Other: 83 Colnago Super Record, Univega Via De Oro
Visit and join the Yahoo Geared Hub Bikes group for support and links.
http://groups.yahoo.com/group/Geared_hub_bikes/