Rick,
Not sure how thick the ABS headliner board is but I use 1/8" and 3/16" ABS in the adapters and cup holders I make. If the ABS you're using is similiar to the stuff I use, and I would think it is, all you have to do is score it with a knife and you can then just bend it and it will break at the score mark. Obviously, the deeper you score it, the easier it will be to break it.
On the 68-72 Cutlass stereo and gauge housings I make, I have to shape/sand edges down to contour them. For this, I use an 80 grit flap wheel on a grinder. Knocks it down like butter.