Gilbert Briggs gives details of his Wharfedale SFB on p.85 of 'Loudspeakers' (4th ed., Leeds, 1955).
Two panels made of plywood, 1" apart, space filled with dry sand. Dimensions, height 40" (no other dimensions given), capacity 9 cu. ft., weight about one hundredweight.
He gives further details but I will not post these unless someone wants them.
He says results of this structure are similar to brick enclosures.
My own experience with filling panels with sand is that it is essential to oven dry the sand first.