I made one from pvc pipe, elbows, pushing springs, and two screws. One pvc pipe could slide in the other so I could make a slide rule type measuring device. The elbows allowed the cross bar to hang lower, avoiding teh lower control arms.
I had measured and made a scale on the pvc so I knew how far the tires were apart, then placed it between the tires, halfway up the front, took the measurement. Then moved the jeep forward so the tires rotated half a turn and took the measurement of the back of the tires. You want to leave the tool betwen the tires when you roll it forward. All you want to do then is find the difference, divide it by two, add that to the smaller measurement, and then adjust it to that.
I believe there is supposed to be a slight toe in (3/16"?) so you want to take that in to account. I'll try and find a pic of my creation so you know what it looks like. I actually just copied the alignment tool that I used in the army, I'm sure theres a pic of one somewhere.
Hope this helps.