I've also been thinking of doing an expanding false bottom for my mash tun, mainly because the opening at the top is 6" and the inside is around 11".
I started thinking about some stainless screen with some sort of spring form edge, almost like the "pop open" car windshield shades (attached pic). I could make some sort of spacers to keep it off the bottom if needed as well. I'm just not sure on how feasible it would be. It would be nice to collapse it, push it to the bottom, and then expand it to fill the entire bottom.