I tried using a dremel, but found it too hard to be consistent without cutting too much material sporadically. With using a medium-coarse file, I can cut enough material away quickly with hard pressure and then use light pressure to fine tune the angles. Here are my steps:
1. Use a flat file with a wooden handle at a 35 to 45 degree angle, depending on cell size. Periodically, place an already assembled battery pack into the slots and look through them from underneath to find out where each cell sits and adjust the sanding accordingly for all six cells to sit approximately .1mm (barely noticeable) above the bottom edge of the chassis.
2. Use a "spike" file (.5mm diameter) to round off the corners
3. Once you have the pack sitting perfectly, use the flat file to barely shave off the sharp edges of each slots short edges (by the belt and the outside chassis edge)
4. Apply a thin layer of CA with a Q-tip to all sides of each slot to make sure the CF doesn't contact any metal from the battery
5. Cut four small pieces of foam from an old bumper, approximately 1mm in width. Trim them accordingly and glue each one to the side of the two outside corner composite battery holder pieces and the two sides of the outside center piece. I've found that this is needed with both 3800's and 3300's to keep the packs from moving from side to side when racing.
I've actually had a pack almost explode after the sharp edges of my original FK's slots wore through the packs cover at two different points. The charged pack started to short, burn the CF and produce a shocking smoke signal.
