Take a piece of steel bar from B&Q, drill three holes in it for the pins, take some 1/8" chain, push the pins almost out and then hammer them in so it's riveted to the bar. Use an existing chain whip as a template (borrow one or find a good photo). Wrap some handlebar tape round the other end as a handle.
The optimum length is a hand's width greater then your wheel radius; any more and it's pointless because your other hand has to turn the rim of the wheel (which limits leverage), too short and it's awkward.