The problem is that the filler hole edge is vulnerable, because it is a sharp edge. No way to protect it really. The proper way would be to machine up a “collar” which has a wrap-around inner edge so it goes a few mm down into the hole.
The “refuelling mat” method is a lot easier.
Couldn’t you just use an epoxy, coloured as needed? Like they use on garage floors or modern table tops.
An update on this:
After some years, they start to crack. I think polycarbonate doesn’t take the UV, or the fuel. A different material is needed.
I had two more made without adhesive, and stuck them with epoxy. The result is good although really needs to be painted over
One needs something tough, because the fueller is going to throw a ~5kg metal item against the edge of the hole 
I will report on Version 3 at some stage But I know it won’t be practical because it will involve gluing the disk (the annulus) to the wing with epoxy, and since the wing is curved it will need to be weighed down until the stuff has gone off.
this is also a very good product that is self adisive
i use it not on aircraft but on a other application were a similar problem exists
http://solutions.3m.com/wps/portal/3M/en_US/3M-Automotive/Aftermarket/Our-Brands/VentureShield/
I got 10 of them made, laser cut from 1mm polycarbonate
Not a success visually, because the paint around the filler cap was uneven (due to having been cleaned off and painted a few times) so the very thin glue didn’t stick except at some spots.
Water also gets underneath, so you need very good undamaged paint underneath. Otherwise corrosion will happen.
I have made a Mk II version with 1.5mm adhesive rubber underneath, which works better but still needs undamaged paint.
Given the material, I would print any “stickers” on the back, there should be no risk of it coming loose at all.
Not all Polycarbonates are UV resistant, worth checking.
I think Jason you must fill at a higher class of airport, because I have almost never seen such mats in use. Maybe twice a year. The man at Pamplona used one. And not where I am based. And carrying them is not ideal because of the smell of fuel inside the plane afterwards.
Regards the material, originally it was to be PET G which I have just discovered is plain old Perspex, which is brittle, so I am changing it to polycarbonate, which is really tough (helmet visors) and UV resistant. This isn’t like a normal sticker; it should last for years. Having said that, I have just gone around the edges of the holes, fixed any damage / corrosion, and primed it with 2-pack epoxy primer. No good sticking this on top of corrosion, obviously.
They will come off but will possibly take the paint with them… especially Socata paint which you can peel off with a fingernail, after the high-tech (probably €1M) paint shop man had a particularly good bottle of wine 
Don’t your fuellers normally have those rubber mats? I find they almost always do. Peter, do you run the risk of yours degrading due to UV? Will you be able to take them off the replace if they do?
