you dont need to use cleft wood, in fact you can make a shave horse from anything, pallets, shuttering ply, recycled wall studs and joists etc. There are certain people in the "green wood supplies industry"
attempting to take lots of good monney for what is essentially a simple and traditionally self made device....there are as many horse forms as there are green woodworkers.
Think of it as a huge pair of pincers, with one side fixed and the other side shut with your feet. My advice would be to do a google picture search of shave horse, get a few ideas. You need to bear in mind your body size. Do you want the thing to be portable/collapsible to go in a car or is it a permanent fixture at home? I would make a few mock ups from any old scrap to get the proprtions right for you.....I personally would recomend the english style (with 2 arms and a crossbar, not the american dumbhead style with just one arm.) ideally go for one with 3 legs (more stable on uneven ground than 4 legs) and splay the leggs at an angle too, this adds to the stability.
There are some assorted shave horses on this thread on the bodgers green woodwork forum
http://www.bodgers.org.uk/bb/phpBB2/viewtopic.php?f=2&t=813&hilit=shave+horse