I guess it's obvious it needs to be wide enough that it fits exactly between the seats. It also needs to be as deep as the seat width, also obvious.
The way it tapers at the front, so when set up as a table there is enough space to be able to pass between the table and the kitchen cabinet, might vary.
When I re-made ours, I just used 3/4 inch birch plywood, framed it with ash and at the front left a 12 inch front with the sides tapering back at 45 degrees -- not very creative, but does the job, at least for the time being.
I would think, if you want yours to be entirely stock and authentic, you could purchase one directly from the Scamp