The caps don't have any UV protection and will become brittle in a short time if left in the sun. Like others have said, to get a replacement cap one will need to buy the entire unit.
Mine went bad on an M7060 in about a month and the dealer gave me a new unit under warranty. Before I went to the dealer, I searched and searched for a replacement lid but to no avail. If someone does find one, please post back where it can be purchased.
Anyway, made a cover for the replacement unit out of some black UV resistant material and that seems to be working out well so far. The cover is basically a "bag" that fits over the entire reservoir (please see attached photo).
Now... what can be done about Kubota's fading paint.... It goes from orange to pukie pink