I've completed three initial mockups using cardboard, junk prints, and hot glue. The dimensions were progressively fed into the Blender model and actual components printed.
Iteration 1. Initial layout with 3 thumb buttons.
At this point, decided to drop one thumb button from design: one joystick plus three buttons is a bit cumbersome to handle.
Iteration 2. Initial layout with 2 thumb buttons and approximate positions for wristpads and joysticks. Actual joystick caps and key caps are used.
Iteration 3. The thumb layout is now final. Experimenting with the joystick offsets and finger buttons.
The same layout ported to Blender: