Mounting a 12v motorcycle light on bike seat rails?
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.