So, after trawling about on the DIY guitar projects areas of many websites, I happened upon the Ruby amp at runoffgroove.com. I nipped down to the local electronics shops, and less than R50 later I had all the parts I needed. Well, almost all the parts. Some scratching around in the pantry yielded the final component - the enclosure!
Above, you can see the volume (left) and gain (right) controls, with the input jack (which doubles as the on-off switch) and the 2" speaker - all 0.5 Watts of it!
Here is the top view, showing the headphone jack (which mutes the speaker) and the "spoon lid", which doubles as the 9V battery compartment cover:
It is a surprisingly effective amp, and had its first public outing last night at a work farewell function. Very impressive for such a simple (and cheap) design...