see if you can find one of these old kirtland bags. it's got a heavy steel mounting
bar that wraps around the stem, the arms slip into reinforced sleeves, bungee cord
from the bottom hooks onto your fender eyelets.

http://www.flickr.com/photos/elkulak/4988436575/
http://www.flickr.com/photos/elkulak...n/photostream/

i used to carry my dslr with 200mm lens with no problems.

you may be able to modify your other bags........