Simplest possible 5V to +/- 10-15V converter (for op-amps)
Take a look at the LT1930.  On page 8 of that datasheet is a 5V to +/-15V converter using a single inductor.  I've built one and it works quite well.

Not exactly "built from stuff already in my workshop", but you can buy the necessary bits and pieces for about $10 from Digikey and it uses about 1.5sqcm of PCB space.