Installing Oval brake lights.

I just bought a pair of LED oval brake lights, and can't figure the proper way of installing them. They come with a rubber boot, but when I cut a hole for them, it just doesn't seem to fit right. The guys that have them, do you mount them in 1/8 or 1/4" plywood? and exactly how big do you make the hole? If I size it to the light, I can't fit the light into the rubber, and if I make it a little bigger, there is too much of a gap around it, so it is sure to leak into the hatch. Also, once I have it in , I can move the light up and down about a 1/4", so it doesn't seem secure. I must be missing something here.... because it sure as heck isn't rocket science!!



