Take four 4 foot lengths of 4 by 1 inch timber and four 2 foot 6 lenghts of 4 by 1 inch and screw together to make two open frames. Use four inch offcuts to strengthen the corners.
Cut two diagonal bracing pieces to fit from lighter timber, using a groove joint where they cross. This gives two strong open frames, which bolt together.
Use 7 inch offcuts of the 4 by 1 inch timber to make two guides at the outer end of each frame. Use 6 inch offcuts from the lighter timber to make guides on the sides.
Make two folding frames out of the 4 by 1 inch timber 1 foot 6 by 2 foot 6. Use long hinges in the corners, so that the frame can fold flat.
The folding frames fit on top of each end of the main frames, fitting snuggly over the four guides. Check that the guides do not foul the hinges.
For storage and transport, the two main frames are unbolted and stacked on top of each other, the guides from one frame interlocking into the other frame. The two small folding frames can then fit into the open box arrangement.