The purpose of the tool is measure the distance between the front of the tyres and the rear of the tyres to determine the Toe In or Toe Out adjustment requirements. The Wheel Alignment Bar is simply an adjustable length Bar or Rod that is extends or contracts to suit and can be locked into a fixed length.
There are plenty of ways to make the tool so this is just my method but the important things to know are you need the bends, it can't be a straight bar, and you need some way to lock it into a variable position. I used a length of hollow RHS about 12mm inside and a piece of Rod that was a very tight fit and if I recall properly I may have run a flapper disc or such over the length to help it fit easier. The important point here is that one piece has to slide in or over the other piece.
At one end I bent the rod with the two corners a fair degree apart however that was to suit my application at the time on the hoist, for normal use on the ground under a Moke you could get away with 50-100mm between bends to clear the gearbox and exhaust. Another option is to work out a point on your tyres that is between 3 and 4pm on the widest edge and then measure the vertical height of this point to determine the best height to all the tool to sit flat on the ground and touch the tyres between 3 or 4pm.
I bent up the two rods in the vice with a hammer and some heat to be a match and then welded one of the rods into the end of the RHS piece. Down the other end I drilled a hole and welded a nut to accept my rather sophisticated locking T-Piece which is required as you will be doing it up and undone with your fingers and using a normal bolt head and spanner is not practical.
A quick run along the top edge of the second rod to give it a flat surface for the bolt to rest on helps stop things moving and requires less pressure on the t-piece to make it firm and that is pretty much it.
In a separate article I will explain how to use you move it to measure the difference between the front edge of the tyres and the rear edge and adjust your steering arms until the front is just a fraction, 1/16 i recall, closer than the rear.