A bit late to the party, as they say, but I simply use an old B.D.B. 27mm screw-in filter with the glass removed
It protrudes 6mm from the front of the lens, so doesn't detract unduly from the overall compactness, but increases my confidence of not damaging the front element significantly
I painted the interior of the filter with matt black paint to eliminate any reflections from the bright aluminium body.
Has the advantage of being able to use the original lens cap, assuming you haven't already lost it!