I'm riding to work, the gym, the store, the bar. I'm wanting to lock my backpack or at least my work clothes etc on the bike. Riding an xtracycle otherwise i'd buy a cargo cache from Otivia.com. But i want something bigger. Just wide enough for a thin bag or a small bag of groceries, about 26" wide and 18-24" high with a lock. The toolbox i use now is a little wide and sticks out a bit much but was cheap.
Any ideas?

I'm open to dry bags that are knife proof, or similar concepts. Just something i can leave cabled to the bike with some relatively inexpensive clothes etc in and not get ripped off.

Hmm. Could you use a cable lock and Cobbworks Oyster bucket panniers? For all I know they are bigger then your toolbox get-up. Yeah, I'm sure someone could cut into them as well, but its the only thing I could think of.

https://www.cobbworks.com/orders.htm

There are lightweight steel nets available for securing backpacks. Not high security, but they are "knife proof".