A number of people commented on the plastic boxes I used with the
locking clips on each side. They are from a company called Lock & Lock
(www.locknlock.com) out of Korea. There's a huge variety. I haven't
been able to find the right sized ones at any US retailer. Target sells
them as set of 5, with only 1 of them probably being small enough for a
box. They are also available on a number of organization sites like
www.organizationetc.com (parter of amazon.com) or houseware sites like
www.heritagemint.com.

I got all mine at the big Korean market in Rancho Cordova, Koreana Plaza
is the name I think. It's on Olsen, right where Folsom Blvd & Zinfandel
meet. They're in the back right corner. Unfortunately, while they have
stacks & stacks of them, they don't seem to keep stock of specific sizes
with any regularity. They just have what ever was shipped last time.
The smallest boxes that I used for the Tower Bridge & Golden Spike
letterboxes were only $1, the Eureaka & Fanny Ann boxes I think were $2.
Also, I know Da Kool Kats founds some at a Japanese shop in San
Francisco. So check out Korean or Asian markets in your area if you
want to find them.