If you're talking about the in-floor storage compartment in the rear of the cargo area, then no, there is nothing commercially available to replace the lid or even the full compartment and make it lockable. If you're really crafty, you might be able to rig up some kind of key-lock to the lid (which is also the floor of the cargo area) and keep it from being opened without excessive force or tools, but it would still be pretty easy for someone to penetrate since it's just plastic.
There is an option for the roof "wet box," which is a Southco brand locking handle (note that you'll have the same "not so secure" issues as the rear storage area, due to the whole box being made out of plastic). This is a good option for certain gear that you don't mind being exposed to some weather, like tow straps and what not. You can also put stuff inside of plastic bags to keep it dry while up there. Unfortunately, the handle needed has been out of production for a while. It can be found from time to time on eBay, or can be found through certain supply houses that have them in stock for a lot of money (close to $100, usually). Some members on this site even post them for sale from time to time. I believe knightrider had one for sale a while back, not sure if it's still available. I came across a stash of chrome ones on eBay a while back that were about $12 shipped. Not sure if they're still available or not, but I'm sure you can find the thread with a little bit of searching.
If you're looking for a REALLY secure option (like for a firearm, etc), you're best and least expensive bet is probably to purchase a commercially available storage locking box of some kind and permanently mounting it down to the inside of the cargo area or under one of your front seats. You can use bolts or something that can only be accessed from the inside when it's open.
