A nice tool to use is a cone wrench found at any bicycle shop. You'll use it on it's side.
5 bucks and you will have a 15mm and 16mm wrench as well. The one pictured is short so you can easily carry it with you.
Ride to the local bike shop, ask if you can bring it outside to check its fit then buy the one that does fit. The fit will be the thickness of the tool's edge. Too think and it will not work too thin and it will bugger the cap. That's why you need to physically try the tool for
fit.
Another point is, do not overtighten the cap. It's got an o-ring so just snug it up. It will stay in place.
