this is made from 8mm Aluminium plate, 700 x 500 mm. The actual grade of Aluminium is unimportant, 2000 series, or 6, or 7 etc. the stiffness is the same for a given size.

you could use steel plate, but that would be quite heavy.

you could use a sheet of plywood, and face it with a thin steel sheet to complete the electrical welding circuit.

My welder gave me this piece of aluminium for free, so i'm not sure about costs... a quick google search found me an online supplier asking £20 - delivered.

you should be able to find a metals stockholder near you in the Yellow pages.

good luck...

 Hey how much would it cost a board of aluminium like that to make a jig?