So I see you don't really need that much bag space. How about a 7- to 10-liter trunk bag? Or maybe even a 7- to 10-liter saddle bag that is securely fastened to your seat? Both are closed through zippers or straps so that it's harder to lift things from them (at least harder than lifting from an open crate).

If you need to keep an eye on your stuff in front, there are the Klickfix options which can probably attach to your handlepost. It's already been linked above, but here it is again from Thor's site:

http://www.thorusa.com/dahon/accessories/baskets.htm

Also, that front rack suggested in post #6 above will probably work.