I designed these when building a lot of shelves. Used in pairs (set the first to the opening size you need, put the second against it (the two back to back), adjust its height to the first, lock it and use the two to position shelves without worry of varying heights.
They can be set from about 9-1/4" to about 15".
These are initial prototypes. I could destroy them, and just have the final version, or I could let you have these for the price of the hardware. Say, five bucks each (screws, washers, nuts and jig knobs). Be advised, if selling these in a set, it would be around $34.00 for the set.
These are in Desert Aire, Washington. That's about 1 hour from the Tri-Cities, next door, so to speak, to Mattawa, Wash. To those who'd ask, no, I won't meet you in Richland / Pasco / Kennewick, or part way. Too many headaches trying to do that in the past. It'd be better for me to throw them in the PLA bin or donate them to Habitat.
Your most likely way of contacting me is, actual phone calls, or email. I don't live on my phone and have been known to forget it in the shop for car for days. As such, I may not see a text for a couple days, if at all.