How about something like this - looks like a mounting bracket on the back of the housing - a little larger than you are looking for but it just might work and for $3.80 from Home Depot not a lot of cash out of pocket to experiment with.
http://www.homedepot.com/p/Blazer-In...55UW/202498003
Then there's this one too - though you would have to open up the bumper hole to 4 1/4" but it is a surface mount and $3.98 out of pocket. Also from Home Depot.
http://www.homedepot.com/p/Blazer-In...KcMHgg&cj=true