I guess this isn't really Festool specific, other than that it was inspired by the MFS, I made it using all Festool tools (except for the tape measure, hack saw and utility knife), I'm going to be using it with the OF 1010 on the MFT 1080, and I'm probably going to be buying an MFS this weekend, partly on the basis of seeing just how cool and useful a tool this concept is.
I took a bunch of 1/4" ply (well, it actually turned out to be closer to 3/16" ply), cut a couple of strips out of it, put spacers that were slightly shorter than the width of the strips (to provide a groove) every few inches, and lapped them out a bit on one end to provide a tongue. Some #10 threaded rod and a little hacksawing on some T-nuts, and I've got a variable sized rectangle jig.
Hopefully this'll get you hobbiests on budgets who are jonesin' for an MFS a little breathing room, but more likely if you build such a thing you'll all of a sudden be thinking about how cool it'd be if this would could clamp to your guide rails and your table and... well... like me, you'll be talking to your dealer, looking for a fix.
A link to some larger pictures and additional ramblings on construction.
