I think the biggest paddle tail I pour is about 8.5-9". I also have a flipp-in tail wangler creature that stretches to about 9"...also I do have cork that can be put in tail from 1/4-1/2 inch size. 

BUT, there is a plastic now that has air bubbles in , that will certainly make your large plastic creatures float...  And probably very even...  :)  I used to take a hypodermic Needle and inject air in plastic as it setup... The crazy things we do to catch these fish.:)

I probably could get the maker of the Hellkat to make a floater. That's a big creature

JD