I have a topeak one as well. It is possibly similar to the one you had on your tour. This is a link to the same model as I have (but newer than mine - wasn't quite as flashy with the embossed branding on the sides and mesh pocket on the top when I bought mine - just the bungee cord)...

http://topeak.com/products/Bags/MTXTrunkBagDXP

It does have velcro loops but really they are only for added security. The bottom of the bag has a plastic track on it that slides into a matching track on the topeak racks. The yellow thing you can see at the end locks onto the rack. It is all very secure really. Then the side pockets unzip and flip down and hook onto the rack to make the panniers. The top has an expanding section and a bungee cord on top to hold your jacket or other clothing you discard!

I have been very happy with the topeak bag. I took it with me on a ten day tour last year (and will again this year) and have also used it when commuting (an A4 folder of work fits in the pannier). It has the shoulder strap so you can just slide it off the rack and walk away with it over your shoulder.