I decided to work on this in the well-lit kitchen instead of in the garage on the workbench (which is covered with car-fixing stuff) and decided to try using a super-giant cutting board to do my gouging. I laid everything out on the floor with this cardboard under the mold to save this pristine cutting board from getting too scarred.
I'm going to remember this cutting board trick. Violins are so small that you could really get quite a lot done on a cutting board if you don't have a workbench handy or just want to work in a new space as a change of pace.
Once the corners are cut down pretty close to the lines on both sides (drawing lines on the top and bottom of the blocks keeps you perpendicular), you can use a file to smooth out the surface. After that, no need to scrape or use sandpaper--the filed surface is just fine (and perhaps better than a slick, smooth surface) as a contact surface to which you'll glue the ribs.