I changed the ignition switch in my old IH to something with a unique key. The major brands of tractors seem to use the same keys. Your neighbors JD, NH, etc... key may well fit your tractor. This will not theft proof your tractor but will help with casual thievery and vandalism + unauthorized "borrowing" by relatives you don't want to loan your toy to.
This isn't necessarily a source, but might be adaptable for use. It also wouldn't deter serious thiefs, but might keep someone from putting something in the tank.
I don't know how many companies offer these, but this picture is from Swagelok.
John Deere 4052R Cab,, Deere 855D UTV, Z920A Zero Turn Mower and assorted implements
"Can't find a replacement lockable cap for the tractor."
I know you won't want to do this...depends on how serious you are to prevent vandalism...
Find a hasp that you can modify (may have to do some welding to lengthen it). The modified hasp be formed to cross over the top of your fuel filler cap.
Bolt it to your tractor hood using self locking nuts on the inside. One can only loosen self locking nuts so much without needing a wrench. Well, there are ways to remove it from one side (prying up on the screw or bolt head), but that's an extremely unlikely scenerio).
Hasp goes over the cap and you use a padlock to keep it secure.
Since you've got a brand new tractor, I really doubt you want to do this...but is it so different then drilling holes to mount a tilt meter?
Yeah... maybe somethin' like that... good thinkin' !
Want to prevent vandalism of stuff in the fuel... & maybe hydraulic system...
Maybe expoxy the stuff instead of bolts... ?
...maybe use a thin braided wire expoxied to the cap... then locked to someplace so it can't be fully opened... ?
I found & checked out the lockout cap from the post... they are made for valves... don't appear to be deep enough for a thick fuel cap... depth not given at web sites...
It's a good lead though !
One other source is Grainger, another may be McMaster Carr although I don't have a chance to look. Just search for valve lockout, or even round valve lockout. Here is the link.
What size cap do you have on your unit? I know we have some of the Swagelok ones at work, I could possibly measure one for thickness next week if you like.
We use hundreds of these valve lockout clamps at work. They are made for gas valve lock out tag out, like you said, not for security. I think someone could just rip one of these apart by hand or at most any screw driver would do the trick. Also they are only about an 1 ½ inch tall so they would only cover the lowest profile gas cap.
The current plastic fuel cap is 2 7/8" diameter and 1" thick...
It's a screw on type... screws over the filler neck...
When on... there's 1/2" space between the cap's bottom on the fuel tank...
The filler neck is slightly smaller diameter than the cap (1/4" - 1/2" ?)