As our budget has been dwindled down to near nothing for our upcoming tour, when my wife and I decided to go with handle bar bags, we decided to give the Performance "Trans-it" brand a try, as we'd heard that they work well, and are cheap ($25 or so on sale).
The problem we ran into when we received the bags, is that the handle bar mount would not fit our handle bars (the diameter of our bike's handle bars was too large for the mounting bracket). Instead of sending them back, I decided to try a little DIY project, which ended up working great!
Here is how the project turned out:
Thought I'd post it here in case anyone else was looking to do the same. What you'll need-
2x 1-foot pieces of 8-32 or 6-32 all-thread (its like a skinny 1' bolt w/out a head, available at most hardware stores)
4x 8-32 or 6-32 nuts with nylon locking inserts
1 can of plasti dip (any color, I used black to match the handle bars)
2 1"x8" strips of thin rubber (I used an old punctured tube, and cut the strips out of it)
First, you need to bend the 2 pieces of all-thread in a U-shape around the handle bars. I wrapped my handle bar in the rubber strip, and starting at the middle, bent the all-thread around the top half, then the bottom. You may want to use gloves, and heat the allthread up to make it more pliable (and if you drop it in cold water to temper it, it will strengthen the bend) but I just bent it cold with no problem.
Once you have your two U-shaped pieces of all thread, using the plasti dip, coat the bottom 2 inches or so of the "U" (just dip it in the can). I applied 2 coats, and let it dry over night. This plastic coating will add some friction to hold the handle bar bag in place, and will cover the threads which could scratch you and/or your handle bars.
The next morning, I test fitted the original mounting bracket, with my newly created mounting hardware. You'll find that the U is too long, so after test fitting, mark where to cut the ends off, then using the hacksaw, trim the ends of the U to the proper length.
The final step is mounting. For my first attempt, I just mounted the bracket with my hardware directly to the handle bars. On a ride, I found that my handle bar bag started to sag, as the mount rotated downwards. The quick fix for this, was to wrap the portion of the handle bars where the mount is, in rubber from a cut up tube. Since this, I have had no issues with the mount rotating downward.
The whole project cost about $5.00 for the all thread and nuts (I had the can of plasti-dip in the garage), and took about a half hour of my time.
Here are couple more pictures: