The crackshaft already has two commercial rubber seals that can be found:
** 25x32x4mm Nitrile Rubber Rotary Shaft Oil Seal Springless Design VC Style type shaft seal (garterless)).
simplybearings.co.uk
** 35x44x4mm Oil Seals for Variomatic Yamaha T-max 500 Injection. It can be found locally in Yamaha motorbike stores or ebay
I tought that a way to improve the seal is to add some waterproof grease, like Motorex Calcium Grease:
https://motorex.com/es-es/bike-grease
Ant BearingMan confirmed that it is what he is using.