Use the inside measurements of you completed seat to determine width if the pieces. Use the height of the seat to determine the height of the backplate and the center 4x4. Cut the 4x4 to 3/4" shy of the height you want the joystick base to rest. Cut out the 3 pieces and assemble the whole stickbase. Use a 1" dowel pushed through the backplate into the 4x4 hole to prevent the 4x4 from moving at the top. Once you have assembled the completed stick base, measure where you want the tabs to be cut in the sides of you cockpit seat and cut them.
Next cut out a square of 3/4" plywood to match the bottom of you joystick. Cut 2 more pieces about 2" wider x 2" longer than the 4x4 crossection. Trace the 4x4 on the backside of the 2 cutouts and make a 4x4 hole. Align, stack and glue them together, then glue them to bottom of the piece that matches the joystick bottom. This assemble rests on the top of the 4x4 and can easily be removed, if needed, to get in and out of the cockpit. Attack your joystick to the joystick base with velcro, or screws if the bottom of joystick is slotted.