RE: Reciprocating Balsa Slotter
I've had good success taking fine-toothed hacksaw blades. Break or cut them into multiple short lengths. Then stack enough together to get the slot width you're looking for, secure them (glue works fine) and add some type of handle (if you're so inclined). You can even put stops on the sides of the blade with wood or tape so that you can only saw down to the correct depth that you're looking for. Nice clean cut, consistent width, and its all done in one pass. No having to go back and clean out the slot.