Whilst I have not tried the SLA OEM Seiko strap
I do have a couple of alternative options
Uncle Seiko Chocolate bar. It’s a bit thin but it is nice and soft
Degaz copy of the MM300 rubber strap. It’s softer and more flexible than the OEM but looks identical
Zulu Diver waffle, this is my preferred option. The strap is thick robust, soft and smells nice. It’s equal if not better than Seiko’s offerings IMO.
I hope this helps.