I’m looking for a grey striped NATO strap for my 21mm lug speedmaster. It seems they are all sold in 20mm or 22mm. Should I get a 22mm and try to trim it at the spring bar?
Something a little bunched looks better than visible spring bar, but you should be able to find a 21mm with some searching. Cheapestnato.com has a wide selection in most sizes.
I've never understood why it's so hard to find 19mm and 21mm NATOs given how easy they should be to make, and how many brands seem to be issuing watches in odd-ball lug widths in order to force purchase of official brand straps.
Omega’s in house natos are 1/2 sizes. So try them ... you pay but they have them. The other option is curved springbars. 21mm lugs? 22mm strap and the curved springbars prevent any bunching and give the extra clearance.