I make mine 18 inches high, because so far I haven't had to put on more than two supers and I don't like lifting heavy weight outside of the gym.
So I make mine from 2x2 treated, based on H shapes. This means two H shapes on either side, then a single cross piece joining the middle of the two outer H's. Morticed, glued and screwed. Then I use flat pieces of wood (so far I have used left-over decking and left-over floor planking) to go down about 6" from the top surfaces. This gives rigidity and also reduces the draught blowing under the OMF.
Forgot to say that a good source of wood is your local estate agents sign company. The signs aren't used for long, and they throw away or burn a lot of 2x2 stakes once they're chipped or broken.