I'm sticking with the Joseph Kousac Super-Firm Suspension block. Really helps keep a higher cadence. Parts of my body will hate me for that, but my legs love me ;-)
Doing in support of the Stroke Association and would ideally like to finish in under 7 hours on my M6L (although might go back to an E if it's good weather). Although, while I might discard the mudguards for a day (just to see if that's where the rattling was coming from), I'm thinking the mini O-bag might be useful depending on weather conditions. My body isn't aero enough to worry about the extra, plus I'm really hoping to spend as much time as possible on other people's back wheels, so aero doesn't come into it, right?