Here is a quick how-to on building turnbuckle mini jacks. These are surprisingly useful little things and can be made in minutes.
The version I built can be any length between 9.5" to 12".
Step 1: Materials
a turnbuckle, I used a 3/8" from Home Depot
two rubber stoppers, I got mine from a chemistry lab supplier
and a long bolt with the same threading as the turnbuckle, I used a 6" long 3/8" bolt
You'll also need the following tools:
Step 2: Make the Cuts
On the turnbuckle there are two threaded ends, one with normal threads and one with reverse threads, remove the normal threaded end you don't need it. To be sure you got rid of the correct end, the long bolt should fit into the empty end of the turnbuckle.
Remove the long bolt and reverse thread end from the turnbuckle. Using the hacksaw, cut off the ends of the bolt and reverse thread end. When finished, they should look like plain old threaded rods.
Step 3: Drill the Stoppers
You will be drilling the rubber stoppers so that they can cap the ends of the bolts you cut in the previous step.
Place the larger end of the stopper on a solid surface, and using a drill bit just a bit smaller than the diameter of the threaded rods, drill into the stopper about halfway. Repeat these steps with another stopper.
Place the stoppers on the cut ends of the threaded rods, insert the threaded rods into the turnbuckle, and you're done.
I used the ones I built to hold a large flat structure up against a window, worked like a charm.