Marc, your Supreme is gonna be alot like my Centurion Elite V (both 21' ). You have a couple options:
1- (The best) custom sac to fill the whole locker and under the seat
2- FlyHigh 750's. (1 in the rear) and another on the floor of the side surfing and a sac (200-300) up in the bow to lengthen the pocket
Most the time I'm slammed with the 750 (rear) and a 400 (with no crew) on my seat with a 400 up in the bow and a 750 on the same side on the floor. My rub rail is kissing the water. The 750 will roll off of the seat but a 400 will stay
I have no problem putting a wave out for 250# lightweights and inland boards Bluelake, redwoody, squirt, and sweetspot do well with mine.
I'm thinking about getting a sac for under the seat so I'm not dealing with a sac on it for when I have a crew and possibly not use my 400 (on seat) and 750 (on floor)
Either way your gonna be happy and have a great time