The rear rack bags tend to be much longer than the Nitto M-12, which is 8" long. All of the rear rack top bags I have seen are at least 12" long.
I looked at camera bags and soft lunch boxes and coolers at Target today. Some of them could work with some modifications -- mainly attaching some velcro straps around the bottom. The main problem is finding one just the right size and shape, about 8-9" long by 5-6" wide.
I also looked at the Topeak handlebar bag linked above and it's not such a good fit. First, it has a wide waistband attached so it can be used as a hip-pack, and there is no way to remove it. It also has a large plastic bracket on the back for attaching to their handlebar mount, and that just gets in the way. Finally, it is wider and shorter than what would fit the Nitto rack best.
Last edited by tarwheel; 02-28-09 at 07:27 PM.