Alright, I just used a document clip to attach one of the tool bags to the bike to see how it'd work.

These aren't huge, but they're more than enough for the ride to work or a quick trip downtown or to the grocery if I don't buy so much.

I may make velcro cover flaps for if I want to use them with the zipper open or just a net that I can stretch over them.

I've got a ton of dry bags for kayaking, so I'm not to worried about whether these are waterproof - I'll just put my stuff in a drybag if rain's an issue and pop that bag into these. And I'll probably put in thin plywood or plastic sheeting to stiffen the back (or maybe aluminum stiffeners like Debw had on hers)

closed up bag: