I don't like keeping track of the 27mm cover, I don't like putting any pressure on that little inner barrel and threads, a lens keeper is a drag on such a small lens (IMHO), and this solution gives a bit of WR (see image below).
This 65mm push-on cap is nice because the price is cheap, concave to allow for optics extension, and covers the entire outer barrel. To achieve a perfect snug fit, add soft side Velcro lining to the inner lip - even a second layer in 3-4 spots if you want it very snug (personal preference, I guess). Cap available here:
B+W #300 65mm Push-On Lens Cap 65-069707 B&H Photo Video
Take note that the image posted at the store website does not match the cap logo design I received...