Good choice. Gap? You want the bag-rider to ride in the ears - not the stitching. I guess either 17 or 19mm will be OK but personally I'd go for the 17mm. The ears are sand filled so a bit of movement to bed the bag-rider in snuggly.
I see you're in North Yorkshire. I shoot F Class at Diggle once a fortnight and I'd be happy to let you try my bag - and there will be one or two other Lenzis there as well - before committing.