Here's a link to a DIY filter I built. The problem you may have is that you have a lot of water....and that water needs to be filtered. So you will not be able to get away with a little, easy to hide filter system. Big pond = big filtration. This one I build is 1/2 of the filtration for my 9000 gallon pond. It works very well.
New Static And Fluid Kaldnes Filter (diy) - KoiShack