Bike Forums - Rear Bike Rack input

So I'm looking for a rear bike rack for my '83 Motobecane Mirage that I converted to a fixed gear and am using as my commuter bike. It does have braze ons down near the rear axle, but does not have anything up near where the seat post is that could be used as an attachment point.

My thread requesting input on trunk bags can be found here:

http://www.bikeforums.net/showthread.php?t=430995

But of course I'm realizing that I need to take into account the rack I'm using as I look to purchase the bag...

My plan is to purchase a truck bag to use on both my commuter bike, but also on my tandem which has a Blackburn rack. I'm guessing that down the road I'll eventually switch over to panniers of some sort for commuting, but want to start off with the trunk bag. Since the trunk bag will also be used on the tandem, I don't know if the Trek and Topeak bags are for me since they work best (or exclusively) with racks made for those bags - even though I like some of those bag options. So as I consider what rack to put on my commuter bike, I am taking into consideration that the trunk bag that I use on it will need to be able to attach easily to the tandem rack I already have.

So back to the rack - what options do I have considering the frame I'm putting it on? In the limited time I researched options, I see clamp on "beam" racks that attach to the seatpost only and then also racks like this one. It appears that it only attaches at the braze on points that I have on my frame, but without any attachment point on the top of the rack somwhere, wouldn't it have the potential to move around a bit if the lower bolts got loose? Am I missing something about how it works?