The best way to set up an accessory utilizing the sail mast hole on a Hobie is with a Ram-Ball.
Ram now makes a Ball Mount with a wedge fit in 1" or 1.5" that inserts directly into the Sail Mast Hole.
look here-->
http://www.ram-mount.com/mount/ram_kayak_mount.htm
and click on
"BASES "
Product Number:
RAP-354U
and
Product Number:
RAP-B-354U