I am in the process of building one.

Using 1" marine grade ply. Two pieces (one 8' x 4' cut in two) both 4' x 4' Cut out roughly 2' wide by about 20" or so deep.

This leaves 1' sides for arms-elbows etc. Currently mounted to folding work stands-adjustable up to 3' high, fit the size I am working with perfectly, about 1' wide, one each side. This will be the sitting bench. Then attach the other sheet above for a standing bench. Just have to do that yet. Then add a protable stool and I am done - appart from lacquering the timber.

Will take some photos when I am up at my friends farm next.

Cheers, Chris