Did some asking and research right after I bought my L4330 - even checked with several manufacturers (Stant, etc.). Found out that NO ONE makes one. Go figure.
You're best bet would be to rig up a locking mechanism yourself. I can visualize mounting a lockable cage around it or mounting a modified locking hasp to the cap itself.
Looking at where the fuel cap is, maybe you could bolt / weld some sort of tabs on the FEL arms and when the tractor is parked, you could place a bar across. But with the position of the cap, I don't see too many options