@172driver – Mulege? 
Like door handles, etc, very few filler caps are made by the airframe manufacturer.
They are mostly generic ones, sometimes modified.
I would suggest taking a close look at the existing caps, taking some photos showing any markings, and posting them here. Usually the manufacturer can be located. Whether a locking version exists is another matter.
There is a lot of stuff on the US market, made by countless small companies. Also a lot of parts used on certified aircraft are made (often by no-name companies) for the homebuilt / US Experimental market and certified under the certified airframe manufacturer’s TC authority. So the homebuilt market can offer interesting options; whether you can legally install such a filler cap depends on who will be looking at your plane
Aquila has lockable fuel caps as a standard for the A210 and A211 series.
I have never tried to use them on a different plane, bit could check with Cessna and Piper tanks if you are interested.
Diameter-wise it should fit, but I right now I can’t say whether the locking mechanisms are compatible.
We normally don’t lock them, though, because in the club environment we don’t want people to break off the keys in the caps.
Edited for autocorrect….
Thanks, guys – advice much appreciated.
In East Africa, almost always, even in fairly remote strips, at least someone turns up and offers to watch the aircraft, over night, or longer.
I guess that’s the solution.
Next problem, hyenas, for some reason like to munch on aircraft tyres, if left to their own devices….branches of thorns? electric fencing?
A small bottle with a wick near each tyre, containing some suitably repellent liquid? Formalin or 0.88 ammonia? Or paint tyres with lion urine, if available, after landing.
I think the most practical device is an electric fence. I am pretty familiar with these (my ex was wildly into horses). The battery and the HV generator can be compact (maybe 2kg) and the 20-30m of the wire and some plastic poles is no worse than that. You can also solar power it easily, with a ~ 30×30cm solar panel charging the battery.
Unfortunately not a starter, Peter.
Any electrical kit just disappears in days – batteries really sort after, let alone the luxury of solar panels. Fence poles, wiring as well.
Looks like lion pee for me – any advice on securing? Plenty of lions though.
Jacko can advise on the lions 
It may be easier to extract pee from a live one, though that brings some dangers…
But hang on. Surely the plane will be stripped instantly. You need a guy with an AK47, as the baseline. That is done with prepositioned avgas drums.
My in house expert from Durban reckons this is not trivial.
Love it, we’ve gone from lockable fuel caps to AK47’s
I wanted to buy a 185 that was once used for gun running in Africa. My guru mechanic looked inside the aft fuselage during the pre purchase and declared “My god we’ve got half of bloody Africa back here, I’m expecting a giraffe to stick his head out any minute”
Maoraigh wrote:
Or paint tyres with lion urine, if available, after landing.
Then you end up with these
