Couldn't you change the filler neck from an "outie" to an "innie" via a piece of th
You asked how and not why since the "why" is your business. Here's my attempt to actually answer your question.
Your L6060 has a fuel neck located in a similar position to my M6060 i.e. low down relative to the engine. If you have any bolts near the fuel cap (see below) it would be very easy to add a locking plate that prevents the fuel cap from being removed. There are three bolts near the fuel cap on my M6060 that could be used to secure a plate that could be hinged and that could be locked with a pad lock.
If I were making something like this for my M6060 I'd weld (or bolt) an angle and hinged plate to the step. It would be an easy project.
If you look at the MX below, the only way to make the fuel cap tamper proof would be to glue, rivet, pin an extension to the filler neck that's able to work with an off the shelf locking fuel cap.
My F-250 has a locking fuel cap but it's the combination lock variety. It has three tumblers numbered 0 through 9 so that's 10^3 combinations i.e. 1,000 combinations. For me, I'm not concerned about fuel being stolen, but as you say, someone adding something to the fuel tank out of spite or envy.
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