Hey I'm with VegasDude on this one. The Marsee tank bags look great and are well built. I have a magnetic 30L size, RM-3031. In the collapsed mode it's 8 in. high and I use it this way most all the time. If you end up at the store and find you have something large to bring home you can unzip one zipper and it pops up to 13 in. high. I bought the bag to carry enough gear for a 5 day trip. If you just need something for "commuting" or day trips I'd go for one of the smaller Marsee's. No straps or buckles to scratch up the tank; it has a rubberized bottom. Magnets make it quick/easy to remove at gas stops.
I've tested the 30L bag up to 180 mph indicated and it stays put. But an 8-10 inch high object on the tank can keep you from getting into a full tuck which makes 170+ mph speeds a little uncomfortable on the neck muscles. I'd go for the shorter Marsee bag, like VegasDude's, if you routinely explore the upper end of the Busa velocity spectrum. Oh, Marsee has excellent cutomer serivce. Most times that I call I talk to Rex Marsee himself. He helped me solve some last minute trip prep problems a couple of times and did overnight shipments (800-293-2400, 909-694-9742).
Larry