This is what I use, and really like it. It is a T bags space saver, comes with rain cover. Velcro's on. 90 dollars

When I start on a trip it's empty, takes little space. Then after a week at sturgis and another at yellowstone and such, it is pretty much full of souvenirs, tee shirts and such.
x 2- that's what I'm looking at getting once I get my TP rack. I like the fact it's super light, and collapsible so that you can fit it easily in a saddlebag when your not using it.
I actually thought I was going to go with the T Bags Accordian (same as the Space Saver, but smaller), which fits the HD contoured TP rack, but I ordered one, and it is much smaller than I thought.
So, I realized if I want a bigger bag, I'll need to get the HD Airwing rack instead. I'm only going to use it for long trips, so I figure I can unscrew and remove the rack when not in use.
Sorry, a bit of thread jack.