Do you have an extra threadless stem and an old beat up handlebar? Here is how you can convert then so that you can mount your handlebar bag behind you on the seatpost.

I used an old MTB handlebar because that's what i had lying around. A drop bar works just as well and may not need to be shimmed to take your handle bar bag mount.

I cut the handlebar down so that it is just a bit wider than the distance my two bag mounting clamps are apart. I used a couple of old crankarm dust caps to plug the handlebar for looks. Handlebar plugs are better but i used what i had on hand.

I also had to shim the seatpost/threadless stem interface because the seatpost was narrower than the stem. that seatpost is getting replaced.

Here are images of the method I used. I think the process can easily be followed from them.

#1 Parts Used by Miele Man, on Flickr

#2 Step 1 by Miele Man, on Flickr

#2 Step 2 Assembly of Stem by Miele Man, on Flickr

#2 Step 3 Stem on Seatpost by Miele Man, on Flickr

#2 Step 4a Handlebar Bag on Stem by Miele Man, on Flickr

#2 Step 4b Handlebar Bag on Stem by Miele Man, on Flickr

#2 Step 4c Handlebar Bag on Stem by Miele Man, on Flickr

This is a very secure system and it allows me to adjust the tilt of the handlebar/seatbag.

cheers