There is no standard size. I have seen Wisner, S&S and AWB holders in this size and they are all different. If you want to make one use any size you like and make the back to fit. The only thing that matters is that a piece of 20X24" film will fit in the frame.
Originally Posted by totalamateur
I can sell you an S&S holder, not sure how the price compares to a VW bug.