I cut my panels using a jigsaw with a fine blade in it - you need to cut the bottom out of the panel, close to the corner, all the way round. Then using something like a dremel tool with a mounted point (small grinding wheel) or similar burr, cut the panel back removing the turned under edge, making the panel flat. If you do it carefully the panel will end up with a radiused edge, and when fitted you won't even see any of the white showing.
As in my other post re. fitting a belly pan, the awkward bit is opening up the front return on the panels where they mate with the belly pan, which is designed to fit with the thinner carbon panels.
Hope this helps.