I like what you sent. They are less expensive than the Ronstan ones I looked at and they have a spring.Those look nice- only other option would be a single block with external cam cleat, like this Garhauer:
Stanchion Swivel Block with Cleat SB-25C | Garhauer (garhauermarine.com)
Not sure which would work better for a spinnaker sheet.
I’m not to concerned as the OEM spinnaker blocks are also attached to the stern rail.The stresses placed on the stern rail are beyond those intended. Perhaps a soft shackle on the stern docking cleat will serve the purpose..