I have this, the 40 Limited, and the 'M' 40mm. I guess I just like pancakes.
They are all much of a muchness, especially the XS and the Limited, hard to tell apart blind testing.
They do sell pinch caps which will fit (i believe they're aimed at video cameras) on the Well Known Auction Site, the little rubber one supplied is just begging to get lost. On the other hand a step up ring will allow the use of filters if you're wanting them.
It would also allow the use of a hood which helps with IQ, only thing being if you hang a hood and filter mount on the front there's not much point buying the XS lens
Apologies for the awful mobile pic but you get the idea.