Google "Secure It" pistol security box or "Gunvault". These are two brands and styles. The secure it box comes with a cable that you loop around something sturdy like the seat rails and then put the other loop over the latch in the security box, close and lock it. If you don't put the loop over the latch it's not as secure. Gander Mt has a different brand beside "Secure It" which might be made a little sturdier, but I bought the "Secure it" and it has worked fine.
Right now Bradis II has the "Secure It" boxes for about $22 and Midway has them listed for $20. Grander Mt's are about $34. Good Luck and let us know what you get.