do they do a 20mm invisible one way shower tray ?
OneWay sloped shower trays are only available in 30 or 40 mm thicknesses, because of their technical specification.
If you prefer 20 mm thick tray, please see similar sized 4-way sloped tray here:
It's default thickness is set to 20, it can also be 28 and 35 mm thick.
I am happy to assist if you have any questions.