Ian, I use an alternative method of cutting out and trimming vac kit parts. I start just as you do with the permanent (and sometimes a paint) marker outline of the parts, making sure I get the ink on both the part and the sheet. Then, I score around the part at a 45 degree angle right at the juncture of the part and the sheet. Light scoring, of several passes, is usually enough to allow me to flex the part out of the sheet (just snaps free.) This leaves the part with just a small amount of excess plastic, as the plastic 'fractures' at the score also at 45 degrees, so the excess has a triangular cross section. I then use a knife, single edge razor and sometimes a mini-plane to "shave" the excess down to nearly the exact line as provided by the permanent marker. Last, I finish off squaring the edges with a small sanding block.
One major advantage to this method is that it precludes the difficulty in getting the even pressure on the part that is required when "board sanding". Since the parts are thin-shell, downward pressure will actually deform the part profile slightly, and when sanding parts such as wings and stabs, it is even trickier as the trailing edges have a much wider contact surface due to the shallow angle.
Another advantage for you might be the reduced arm movement, thereby lessening the aggravation on your back (?)
Sorry this is too late to be useful to this project, but perhaps it will be of help in the future.
Regards,
Robert (board newbie, modeling old-hand)