Ardusub Custom-Frames

I want to build a submarine with
-1motor
-2rudder Servos for stearing
-1Kamera Tilt servo
-relai controllt pump for balastsystem

  • Mateksys H743-WLITE Flight Controller
    -Raspberry pi 4 8gb
    -and some other things
    But how can i set up the frame?
    And the custom fuktion with the balast system how can i get it to work?