Does your FP machine have a doona wash? I have a FP, and it is somewhere around 8kgs (can't remember what it is now) and I throw our king size doona in there on the doona cycle. It fills the water to a high level so the doona is fully in the water.
I just go downstairs to the machine, about half way through the cycle and check on it, sometimes I just push the doona back down into the machine a little, but it seems to work well for us.

Nic