Here's a link that shows you how to make and use an ammeter
shunt.  http://www.motorcycleproject.com/motorcycle/text/shunt.html

It should be good for maybe as much as 200 amps for measurements a
few seconds long -- if the clips make excellent contact on each end.

Because it's made of copper this shunt is highly
temperature-dependent -- for every 10C/18F rise from room temperature
the reading will rise by ~4%.  But it's cheap and easy and will
certainly give you a good ball-park reading.  For accuracy I would
also add a small wire sticking out of the back end of each clip to
attach the meter to.  It's the length between attachment points that
matters.  To slow the temperature rise for high currents you could
make it serpentine and clamp it between a couple hefty chunks of aluminum.

To read lower currents you can double the length between measuring
points, so that you get two mv per amp instead of one.

Yours,
David