These pics give you an idea of how it can lay out. You won't get much more than 400# under the seat no matter how you do it.
If it was me, I' put a bag in the ski locker too. When you aren't doing slalom, fill the bag and put the skis on the floor behind / beside the driver seat. Then you get 800# in theback and the only thing on the floor is the skis. You could also put the skis in a tower rack if you have a tower.
You will probably be fine with 400# in the rear and 400# up front. It might not be exactly as big as you want, but you could have a decent wake while still having storage space. If you find it isn't enough, put the skis on the floor and a bag in the ski locker.