Does someone have a neat trick or (homemade) tool for cutting diagonally through thicker balsa (say 4-6mm)? I'd like to be able to cut tapered balsa spars (for why, see my DrI thread) but I'm not having much luck cutting them square with my exacto knife. I always seem to end up cutting at a slight angle. And trying to sand them level with a sanding bar after cutting is next to impossible.

So imagine this situation. I need to make a balsa spar that tapers from about 3/8" down to 1/8" over a distance of about 20 inches. The taper is only on one side of the spar (thus forming a greatly elongated right triangle). It's critical that all the sides be square all along the spar. And I need to be able to make four absolutely identically tapering spars.

Any ideas?