I hammered together a wooden one 10 years ago and have used in on two interstates many times, and for many washes, and left bikes on it up to a week at a time, and have complete stability and confidence in it, and it is still going strong (though it's kinda dirty/oily now). I did use pressure treat for the two uprights, but just soft pine on the one inch bottom (the two inch, 2X4 bottom is harder to get on and off the jack under the bike, and is unnecessary, since the base is just squished between the jack and the bottom of the engine).
Just so you know, some guys have got the metal bolt-on kind (but not the permanent one), and have forgotten to take them off and went riding with it on there, and that is a very bad business.
http://www.herberts.org/wayne/valk/lowtechlift.htmI copied the last one in the link.