Actually, those stem bags (and the front bag) are cheapo camera bags that I bought for about $20 total at one of those discount/closeout stores. They're attached with black zip ties - which are cheap enough to just clip off on the oft times when I need to remove the bags.... but some velcro straps would work just as well.

I used some industrial strength velcro on the tops of the bags (and the back of my electronics) to secure everything (and believe me, they don't budge a bit!) and when it starts to sprinkle, I have 3 elasticized, see-thru shower caps that fit perfectly over the two stem's and the front handlebar.

Here's a closer look: