I really like your instructable. It shows that people are indeed using their bikes for transporting cargo, and not just for recreation. I do have a couple quick ideas that may help you to enjoy this more.

Instead of using a large bungee cord, try using several zip ties (I can get 40 for about $1.50). If you evenly place them along your handlebars, you will pull your crate closer, thus making it more stable. Also, try making a cut out for the bike stem's large square head to pass through. This will also pull the crate closer. Try hooking to other points on the bike it you still experience some wobble. Just bear in mind that if you attach to some places, the handlebars will be stiff or unable to turn.

I like what you have done. This is the perfect start to the "perfect" cargo bike, giving you the freedom to carry any and all that you want.

Keep up the good work! I look forward to seeing your next project. I've added a few links to other instructables that I have drawn inspiration from; hope they help stir up fresh ideas for you, too. (The one of the trailer shows my rig in the comment I left.)

If you have any questions for me, feel free to ask. I'm on this sight almost everyday.

All the best to you!!!
Harold E. Wilcher

http://www.instructables.com/id/A-bike-trailer--Thats-unheard-of/
http://www.instructables.com/id/PVC-and-Milk-Basket-Bike-Basket/
http://www.instructables.com/id/The-incredible-Rim-Rack/