Mounting a 12v motorcycle light on bike seat rails? So now that I built a 12 volt (more like 11v) battery for my bike I would like to accessorize it with some 12v lights to get a unique look. I'm looking at motorcycle tail lights, but they all have two parallel bolts coming out the back 82mm apart. The rails on a bike seat seem to be about 50mm apart. 

Is there something I get at the hardware store to jerry-rig a solution?



Back of the bike seat: (two horizontal rails 50mm apart with an angle at the end). Maybe I could zip tie in a ubolt pipe clamp somehow?
Similar lights are often fitted to pannier racks, especially on e-bikes. I've had a plastic part with the right holes that would be easy to modify, but it came with a topeak rack. 

It wouldn't be hard to fabricate something. The easiest way is probably to start with a saddle mount for a water bottle cage and make an aluminum plate. Some sellers of offcut sheet metal on ebay will cut to size, then you'd only need to drill the holes, but you can cut it with a hacksaw. I'd probably use 3mm plate. 

I assume you've been sensible about light levels and colours and won't dazzle or confuse anyone.