Make these holes for the filter section wall.
(a) first filter hole - about 6 inches below water level
(b) Third filter/ chamber - about 6 inches below water level - for extra pump to pump water from filter to pond (contingency)
(c) Third chamber - in the middle for wiring./ air hose etc (contingency)
(a) the first is to put a surface skimmer on the main pond and the flow is into the first chamber
(b) Second is for another pump in the event your pump for waterfall does not give enough volume to turn around the water cycle in 1 hour
(c) third is to facilitate wirings/ air hose if necessary whcih u can have it mid way or above the water level. Pls note if mid way, u may have to get some rubber sealant to protect water to flow into the last chamber...as its only meant to facilitate wiring/ air hose or if u use uni ring, this is where the hose can go through since u didnt do any upfront.