Rather than commit to even more money, I thought a bit about making such a thing.
My first thoughts involved routing a dovetail down the edge of some 3/4" thick lumber, and then setting up some sort of cam system in the end of each board to lock in place. Then I thought about buying a bunch of T-Track, and gluing it to the edge.
The spacers are 1/4" shorter than the width of the main pieces, and the ones on the end protrude out into that space, allowing one to ride inside the other.MFS can do that this can't, but as a simple way to route rectangular spaces or keep rectangular frames in place, this seems to be a decent first pass.
This one was glued up, I might consider doing the next thing out of two layers of 3/8" baltic birch (this was a birch ply, but it's only got the sheathing and a core) and routing out the gaps, then mating the things. In fact, you could route 'em 8 wide, run a dado bit down the cut lines, and cut them after the forming. One less set of things to align during glue-up.
And depending on what stock you're working with, I'd make sure that you're the full thickness of your most common stock, because this plywood was slightly thin we may end up with some cardboard shims occasionally.
I'd also... either make the sections slightly narrower (maybe) or splurge on some extra threaded rod (I cut 12" lengths in half) so I could do a tongue and groove carrier on the outside edge, rather than just depending on the washer to provide the stop. And that'd make the pieces reversible, too, which would eliminate one source of confusion during assembly.
My original plan was to do lock-nuts backed up against cap-nuts, but the rods were just a touch too short to do that and I drove the rod through the cap-nuts when overtightening, so I've just got a lock-nut and a washer on the far side there. I might just take a file to the nut/rod and use a straight screwdriver for adjustment, but there's enough tension in this thing once you start to lock it down that hand-tight has been fine so far.
Of course if you can find > 6" quarter inch or #10 bolts (Allen head would be great), more power too ya!
And one of the things this showed me, should I go for the full-on MFS, is that it doesn't have to be terribly big to be useful. When I started putting this thing together from a 2'x4' sheet of plywood I thought about doing two 3' segments, but the 2' segments are large enough to align a decent sized drawer, and anything longer would be really unweildy for smaller stuff.