Unfortunately, the Seiko ratcheting clasp is only available in 18mm, AND it is very expensive too.
For certain divers, Citizen has also made a ratcheting clasp (BK-H1492), this clasp expands to the back side, like the solid ratcheting clasp.
This clasp is a non-branded copy of the Citizen-clasp, it can axpand up to 1.2cm and has 5 positions. NO adjustment holes, only the ratchting expansion. Big advantage is that this clasp is available in both 18 and 20mm width. The low cost is also nice. This clasp is less robust than the solid ratcheting clasp, it is lesbulky and 2mm less thick. In fact, it has the same thickness as a standard Seiko clasp.
Length is 42mm, 54mm fully expanded. Heigth/thickness is 6,5mm (the solid ratcheting clasp is 8,6mm high/thick).