Rear dynamo light (tail light) or reflector without rack?
Rack upper eyelets of that sort don't have a standard distance between them, so there are no off-the-shelf brackets that would attach to both of them and center the light, if that's what you're thinking.
Busch and Mueller makes a bunch of brackets that let you mount 50mm and 80mm generator lights (yours is 50) intended for rack mounting and stick them to other things.

You could use this one to attach that light to one of the eyelets, but it might stick out too far to the side.
If you want to do it in a way that's free and made out of trash, one thing I've done several times is take the plastic 50mm/80mm light bracket that comes with Topeak racks and also has the vertical two-hole pattern for reflectors, then bolt it to a seatpost reflector bracket.