I use a lot of the flank bulbs on my car, from the regular ones to the high power ones, the same ones you speak of, but I didn't have to shave anything down for them to fit, just pushed a little harder and they went in.

 

I've never had a problem with a flank LED.

The thick PCB was from a forward firing 4-SMT White LED (80 lumens) that was made to 2.5mm thickness (too thick). The mirror lights on the Cobalts with a sunroof are perfect applications for this type of LED. It's a shame the Chinese manufacturer is an idiot.

 

What particular bulbs do you guys have for your license plate? I assume it is also a 194... I guess where it is only pointing down a flank would be ok?