When I had first gotten my aerotrunk I used a 3/4" piece of pvc to go horozontally on top of the seat that had a T in the middle to lock it to the headrest mount or where the seat mesh comes together in the back. Of course I still had the rack back there so it wouldn't have hit my tire anyhow but it did wag around a bit.
When I locked it down to the rack it stopped wagging and eventually when I tore the straps and webbing due to it getting caught in the rear wheel I got the ortlieb quick release along with a double layer of coroplast stiffner to lock it to the rack. I currently never remove the aerotrunk because it is pretty awkward to carry and it sometimes took a minute or two to get the quick release, two carribiners and the velcro through the D ring attached to my rack/seat/headrest to hold the thing. I have a bag with my clothes and other stuff I put it in and my laptop bag and just remove them and leave the aerotrunk on the bike.
 
 
I remember seeing this link over at the bacchetta site which may be alot lighter then my rack/ortlieb combo. http://www.bacchettabikes.com/forum2/photo.asp?photoid=222&asorttype=6&a page=1 I have a set of seat struts for a bacchetta laying around that you are free to have if you want them to play around with for the aerotrunk. You could attach them a the seat (or fender mount if you have one) and then bolt them to something that would be mounted to bottom of the aerotrunk.  

The seat struts look like a good way to go, or the Bacchetta Back Rack. I used that on my daughter's Agio to support the BrainBox for a while until we did something else. It's light and flat and maybe has a good chance of supporting the Aerotrunk pretty well, depending on seat angle, etc.
 http://www.bacchettashop.com/sess/utn;jsessionid=154ab078cd9e155/shopdata/0010_A ccessories/0030_Racks/product_details.shopscript?article=0010_Back%2BRack%2B%3D281007%3D29