Honestly...unless you were a welder/fabricator with a shop/tools at your disposal, the hundred dollar mail order jack is probably the least expensive option.
While it would certainly be a lot more fun to spend a few hours scrounging up parts at the junk yard, and then finding other parts at the scrapyard, and then dreaming up a way to make it all work together, it would likely cost much more in the long run.
At some point, a couple old screw up stabilizers would get the job done, and leave a bit for the next camping trip... but of course that takes all the excitement out of creating it yourself...
Are there any RV dealers close by? A minute or two spent under a rig with your camera would give you a pretty good start on what you'd need to make your own...
Good luck, if you come up with a really easy solution, I'd be interested, crawling around to set stabilizers isn't much fun.
Rob