Peugeot 308 windshield washer fluid tank cap (maybe other models/makes also)

Print with TPU or some other flex/semiflex material. Go slow with logo, or preferably print the nologo -version without the nitty gritty small details.

Test fit showed a good tight,but not too tight fit.

edit: 9.3.2018 - added a version with a hole for releasing possible pressure and a lip for ease of use ( the lip adds the amount of support in print so use if necessary, works fine without it )

edit 26.2.2019 - no signs of wear or degradation of TPU although i splashed quite a few times some alcohol/soap mixtures over it during the winter.