I've got a couple possibly crazy thoughts...

I might try getting some thin armaflex or other insulation material from the hardware store and cutting it up a bit into pieces that could be good wedges. We put armaflex on pipes carrying 200 degree water, so I'd think it would be comfortable a couple inches from an incandescent bulb. Another thought (not having taken apart a wall-washer recently) would be to pull the trim off the can light and use a few sheet metal screws or something to affix the aperture to the frame so it can wander and vibrate, but I don't know if that'll work structurally and it will make it effectively impossible to adjust your aiming later.