Pat Barber wrote in message ...
A sheet of carefully selected MDF is gonna be
VERY straight. 1/2" or 3/4"... Rip a piece to the
size you wish making note of factory edge.

You "might" find a metal stud that is quite straight
also. These are a little harder to come by but can
be bought in most buidling centers.

If you go this route, I'd recommend the heavier gauge metal--it won't
flex in the middle.

Dan