I like my Novarra Transfers. I don't usually carry quite that much load - but I have. I bought an old iron at an estate sale on my way home one day - my bag was already loaded and this sucker weighed like 10lbs or something. I use them every day and they are holding up great. Kick-flix mounting for extremely quick, secure and virtually universal on/off performance.

edit: crapper - can't find them on REI's site. Hopefully they're just temporarily out of stock - I love mine.