Unfortunately there is some tedious do-the-math on the need for heat-sinks, how big they need to be, etc.. I figure you'll probably need more than 1 bridge rectifier anyway, so why not use some scrap metal to double-duty as a mechanical mounting platform and a electrical heatsink. As shown in the photo, I use "scrap" 1/8" aluminum sheet and mount the bridge rectifiers with #4 machine-screws into threaded holes. But whatever you have lying around will work. The purists will say to use heat-sink paste/compound (the outrageously priced white-stuff between the bridge and the heat-sink plate). I can't argue with that but you'll get plenty of benefit by just firmly attaching the component to any metal surface!
Note that unless you have some funky operation, the voltage dropping diodes/bridges will never operate anywhere near their limits. That is, this is dropping voltage (burning power) when the engine is going DOWN hill meaning power demand is by definition lower. Stated differently, if you're burning anywhere close to 10 Amps going DOWN hill, then you'll have to trust me that you've got bigger fish to fry! 
