i use a TDC brand capper for my #11s. it looks similar to the one sabotloader posted, but has a spring on each side of the tip unlike the traditions model in the picture. i don't think it's ever let a cap fall out in the 4 or 5 years i've owned it. i had the traditions capper for a while and it would always seem to lose them.
traditions does make a short "field" capper, wish TDC offered one.