Weighting x-5

I have a 99 MC sporstar(same as 2000 x-5), i am currently running 1200 lbs in side sacs. At 18.5 mph the wake is nice, big and rampy, but i like riding at a higher speed, around 21-22, at which point the wake flattens out a bit. I assume this is due to my lack of weight in the rear(so once the boat hits 20-21 the rear end lifts up out of the water too much). I want to add as much weight as i can to the rear trunk of this boat, however i do not know what the most efficient way of doing this is. I can only use water as i'm already over the tow limit on my car as is. Does anybody have a 2000 x-5 or 1998 prostar with weight in the trunk. I am wondering what i can fit in there sac wise, it isn't that big an area due to the gas tank at the back of it. I was thinking of trying to wedge a ski locker sac in there. I don't think a full fat sac would work(too wide). Anybody know of anything else, other than a custom sac.