2 pieces of sheet metal, about 24" long & 12-14" wide. Bend a ~45* angle along the longer sides, and cut 2 slots 2-4" above the bend on both ends. Lay these against the outsides of the tires, and use 2 tape measures in the slots.
These can easily be made if you have access to sheet metal, a brake, and something to cut the slots. Otherwise, look at racer supply websites for "Toe plates" and spend upwards of $100 on pre-made plates.

Note: these are only useful in setting the toe. A camber/caster guage is still needed to adjust the camber/caster.