[BrendanP]

I expect the tool would be something like that, but I think that is designed to grip the top of a metal can oil filter. I have seen sets that have multiple sockets to fit various filters. For the cap on the Hyundai it would be useful to know how many flats there are around the cap, what the distance is across the flats, and how deep  the socket has to be to fit over the dome of the cap.

It also looks like there is limited clearance around the cap so the socket has to be slim enough to get over the flats on the cap.