I tow both a 10 ton tag and a 12 ton 34' GN trailer with my pickup. I have a Greeenlee tool box in the bed that measures 2' x 2'6"x 4'6". It sets just in front of my GN ball. That is a secure box. IMO that Excursion is not secure at all. If anyone is serious about wanting your stuff it's just a broken window and 30 seconds away. Sure it will stop the casual snatch and grab but not a hard core tool grabber. My box is chained into the bed and doubled padlocked lid. It would take a key, a torch, or a chopsaw to get into it or steal the whole box. It won't stop them but it will slow them down a bit.
I'd take my GN over my tag almost anytime. Tows nicer, can load more up front weight wise, and if space is an issue you can always mount the box on top of the GN. Down side to a GN is they are heavier. My GN empty is 8,200# and my tag is 4,100#.