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
Waiting for "someday" will leave you on your deathbed wondering why you didn't just rearrange your priorities and enjoy the time you had, instead of waiting for a "better" time to come along...