If no drawing is available, how about measuring the mandrel onto which the Faceplate will fit?
Length of thread (You already know the size / pitch )
Diameter of register (If any)
Length of Register
Diameter of Faceplate can be whatever you wish; withing the limits of the maximum swing
Thickness, again whatever you wish. Sensibly, it will probably be not less than 1/4" and not more than 1/2".
Probably 3/8"
How many slots, and the dimensions, will, again, be your choice., possibly inspired what pictures you may have seen, in catalogues, or websites, such as Lathes UK.
Howard