In the desert world, a rear amber facing light is a necessity. Other than that , I've never had the need for a rear facing light.
But, if I did I would think that making some sort of removable bracket that would facilitate mounting a light to the hitch tube might be a good idea if you didn't want it on the rack.
I don't see the logic of not mounting it to the back of the rack on the last bar though. It's not going to be that high and if something is scraping it off, you are probably going to have a lot bigger problems than losing a $10 light.
Badercarma , I think the feedback you are getting is based in the fact that you basically ruined the paint on both sides of the hatch to booger weld two $10 lights when there might have been a cleaner solution that might have violated your X a little less.
But hey, seriously: It's your X and you mod it however you like that fits your needs. As long as your happy that's what matters.