Balsa, for all it's strength to weight benefits is also a very brittle wood. It doesn't mind sharp cutting blades but will shred if attacked with anything with anything but the finest of teeth; that's why razor saws have such fine teeth. I use a round file to sharpen one end of the tube, but don't strop the edge, instead leaving the fine serrations making them, in effect circular razor saws. The only real drawback is I do have to periodically re-file the edge back in as brass is not a real hard metal and dulls fast.

Smaller ones I can chuck up in my drill for boring through real thick wood or thick stacks. For larger holes I just turn it with my hands, easing up as it starts to break through, or even turning the wood over and finishing the cut through the 'back'. The regular hole saws I used for the rudder I turned by hand for better control. The holes do not end up as clean as they look in the pictures, there is a little feathering, but that is easily sanded clean.

P.S. It is a kit that I am modifying (the original design is just RET). The Skimmer has always to my knowledge been a kit, but hasn't been manufactured for years now. http://aerocraftrc.com made them for Hobby Lobby at first.