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Good to know, thanks. Unfortunate that B&H don't stock them, though.
I was considering trying the B+W XS-Pro ND: https://www.bhphotovideo.com/c/product/1298665-REG/b_w_1089178_xs_pro_58mm_nano_mc_803.html
A slight vignette I could probably live with, as long as it isn't a hard vignette.
One Vietnamese company seem to make a holder for 100mm square filters, unfortunately it seems a little tricky to get. NiSi also make one for the 10/5.6 model but not the 12 or the 15, annoyingly.
NiSi is probably interested in making a holder for the 15mm and 12mm, but they need someone to send them the lens first (for a few weeks) as I did with my 10mm 5.6.
The person who lends his lens usually gets the first holder and maybe even a filter or two (don't quote me on that!).
In case any of you is interested send me a PN, I will establish contact then.