Also: the fredly trunk.

Made of 1/8" plywood and some fir scrap, I think it weighs a little over 2 pounds. Glued and stapled together. The lid slides back to remove, and the notches fit snugly enough so it doesn't rattle or fall off. Also to reduce rattling, it mounts over a couple slices of 1/2" scrap foam rubber, so the box doesn't actually touch the metal of the rack it sits on. If the load is too bulky to fit under the lid, just take it off and stuff it in alongside. The thing easily holds more weight than I care to carry up that high on a bike. When empty, it's still kind of noisy, so when I know I'm not going to need to haul anything for awhile, I take it off. Easy enough (takes 2 minutes, max) because it's mounted with three wingnuts.