Ben
On that Metabo SXE400 3 " sander, couldn't you just take the backing plate off and put it on a disc sander and take it down to 3"? I looked at the backing plate and it looks like it could be done that way and also use the foam pads for the contour of the lights.
Take a look:
http://www.contractorstoolsupply.com...ad-p-4017.html
That way there is no chance of acadently hitting a painted bumper if not thinking....... I would even think of taking it down a tad smaller than that.
Take a look at yours and let me know what you think...
Just a thought.
Don D.