First don't mess with the size: 20 between the lugs / 18 mm at the buckle.
Then, keep in mind that a thin strap is in order, as the case is really thin.
I would personally see two straps:
- A thin crocodile, without stitching. Black is better. If you want stitching, a clear honey may compliment the black croc, and match with the indexes. Not too brown.
- The must of the must, the Creme de la Creme: a thin Cordovan, a black one, without stitching.
Have fun!
Best,
Nicolas.