I would recommend a Skip Frye Flex by True Ames in 8.5" - I have a single fin daily-driver that’s pretty darn close in dimensions (we all know what hides behind an assumption like that) and this fin is magic in it. The flex is progressive and the lower part quite stiff so you get that little bit of zing from the tip with plenty of drive and no mushiness like some of the super-flex models.
Oh yeah: finally picked up the board mentioned in OP. Will post PICs
ASAP, along with PIC of first single fin I plan to test on it as
soon as I slap on some traction pads and find appropriate test waves. Will also be trying it as a twin with trailer and a 2 + 1.