Sounds like you need a dip tube. You can make one similar to this if you are so inclined:
Depending on your dimensions, you may be able to purchase one that will work for you. I have a 10 gallon Blichmann Kettle that has a dip tube that sits about 1/4" from the bottom, stir up a whirlpool by hand to settle hot break and hops in center, get nearly complete drainage.
Maybe someone with a larger kettle can comment on a better idea.