I had a rack once which was rated for 70kg: http://bikesandbeyond.ca/itemdetails...gId=39&id=1003

Mind you, i'm not sure that the mounting bolts would have been good at that weight or that the eyelets would've taken it, but it didn't matter:

The rack itself failed under a load of about 10kg loaded on top. With cargo sitting on top of the rack rather than the sides, the problem is that the cargo can create lots of lateral stress on the rack if you/it moves. I would try and find something which is reinforced against that movement, or see if you can reinforce it afterwards.