I have never purchased from this company, but just found the site the other day when I was thinking about the same thing. Security Briefcase
I have also seen a small lockable box with cable for roughly $35 at places like Fort Liberty Firearms in Indpls.
There is another company that makes Jeep specific aftermarket items, and they have an under seat option as well. Found them through someone posting on INGO about selling some Jeep accessories.
Maybe those ideas help a little? If you find something you like and use, please post back. I'm also considering something similar and am shopping for what other options might be available.