One range I know uses tops of 3" cast concrete, in a Tee-shape and wide enough in the base-leg of the Tee for two shooters if need be. Weatherproof, too...
If they absolutely must be portable, make a three-leg support. That way, the ground does not have to be absolutely smooth. And bolt the frame together; no nails. Use screws on the "deck". Use treated wood and two coats of deck-paint.
As far as the shape and dimensions of the deck, sit at a table with your rifle on sandbags and figure out how much room you need, and what shape is best.
I built one, years back, which was a 60-or 75-degree triangle with the narrow-end top chopped off to maybe a one-foot width. Worked fine.
FWIW, Art
