I've had my racks built with an extra 1/8 gap. I setup the FW directly on the shelf and then I put in a 1/8 particle material (its reminds me of cardboard but its not...can't remember the exact name). Anyway this keeps my FW in place and even helps spread the heat a little better. I didn't like the way the tubs where sliding directly over it and this was an easy fix.

I would also not to worry about FW with a quality controller. Honestly I'd be more worried about those zoomed heat pads that are not on a automated tstat.