Way back in the dreamtime when I first started aeromodelling, I bought a set of X-acto balsa gouges for hollowing blocks.  They were formed from strip steel into a range of interesting shapes, were sharpened, knife like on one side and fitted into the round hole in my large X-acto handle.  I did not use them a lot at the time, and when next I considered using them they were missing.  (They were here twenty years ago, where could they have got to?)  So seeing a similar set in a local hobby shop I bought them to expand my block hollowing capabilities.  It was only when I finally tried using them, I realised just how useless they are for their job of hollowing block balsa.  They were awkward to use, blunt, and virtually unsharpenable, leaving a finish as though the wood had been chewed by a rat.  In fact a careless rat with blunt teeth. 

Feeling rather let down by this, I put my mind to the problem of hacking the surplus wood from the inside of a cowling and came up with the solution.  Still using the same principle of a shaped blade in the knife handle, I made my own blades from that very useful stuff, K&S brass tubing.  The tubing can be sharpened easily by running a number 11 scalpel blade around the inside.  By sharpening the cutters on the inside (Opposite to the commercial design) and soldering them to another small tube that fits into the handle, I ended up with cutters that really do work and can be re-sharpened easily.  Have a look at the (admittedly primitive) sketch that you should find nearby.

Here is a simple (if crude) sketch to show what I mean about making your own balsa gouges from brass tubing.  The commercial type is made from a strip of hardened steel, but with the cutting edge chamfer formed on the outside they cut very poorly.  The brass home made type cut much more easily and cleanly.