The rack is a Surly "8 Pack" rack. It's come in handy on numerous occasions - just right for a load that's too big to lug in my hands, but not so big I need to trailer it. And - I already have a box that attaches on top of the rack. I'll probably use these boxes more often, because I can mount 'em in 1 minute.
The boxes are "30 caliber ammo boxes" from Harbor Freight. The regular price is $6 per, but they frequently go on sale for around $4.
I spent two years trying to find some specialty plastic hooks, that would attach to the boxes, and then hook over the 10mm tubing of the rack. (You'd think there would be something - after all, there are various panniers, etc., designed for such racks. But I never found something designed specifically for that size tubing.) I finally settled on some metal "wall mounted hook fasteners for ceramic tile display." They were just a bit too big - could've set myself up for some persistent rattling... but that was before I used some old inner tube material over them, for padding between hooks and rack.
I also found some nice little nylon hooks that I put on the outside, so I could lash something larger to the tops of the boxes and rack. At some point, those will come in handy.
Total investment - north of $10, south of $15. Not bad!
I expect these boxes to come in handy for "day trips." I could stash a sammich, granola bars and a couple apples or bananas in one... a lightweight rain jacket and my camera in the other. (Or alternatively, I could load 'em both full of 30-caliber ammo!!)
Detail of metal hooks: