I found a pack of 36 x 15mm wooden blocks in the crafts section at Dollar Tree for $1.25 USD (they really need to change their name for truth in advertising).
These can be left as is for cardboard boxes or wooden crates, or painted for more SciFi-like containers. Some tiny decals or other markings, if repainting them would help sell them a bit better.
White, or bright colors would work well, especially if decals were applied. Aluminum or steel spraypaint would work well too for a metallic container look.
So, at less than 4 cents a piece they're a great deal. Looks like I should go back and pick up a couple more packs for my Starport scenery.
Most are indeed 15mm x 15mm, which works out to 5' x 5' in 15mm scale, so very similar to 20th/21st Century large IATA containers frequently used in the air shipping industry.
I only bought one, since I wasn't quite sure of their size, but can definitely use more as loose containers and/or grouped in stacks for storage.
Creating some small, matching pallets to go underneath them will help sell them even more – pretty easy to do with some strip wood, or styrene strips and a little time.
Then of course, I'll need to find a forklift, since I can't recall if I bought one of those or not.