Do a semiraised pond with walls 18-24" above grade. The the filter pit only needs to 12" below grade....plus the depth of the concrete floor. I put a 12" think concrete floor in our filter pit and did 1/2" rebard on 12" centers.
In my opinion semiraised ponds offer so many advantages. Less digging for both the pond and for the filter pit. You can sit on the wall and feed the fish and observe them. You do not have to get down on hands and knees to net or bowl. The raised walls also keeps out wind blow debris on the ground, pets, little kids, dogs, and other unwelcome guests.
The older you get, the more you appreciate a semiraised pond design.