Update 2024-09-26: STL file added + revision of left base plate.
Description:
This snap-on mod allows the Xbox One controller to be placed down and played from top-down.
This mod was inspired by and is a remix of Akaki's Snap-On Xbox Gamepad HOTAS Joystick:
https://www.thingiverse.com/thing:4612448
The contoller can be snapped on the base/mount assembly and placed down on the flat surface. The joysticks and buttons are extended outwards and enlarged for easier reachability. The levers for the RB, RT, LB & LT allow them to be pressed from top-down.
As I am not a gamer I think that this mod can be further optimized. I would really apprecriate your feedbacks and shared makes. Thanks a lot! :)
Assembly:
Depending on the tolerances of your printers you might need a file to work on the connection points like the hinges or ball joints.
Optionally glue can be use for connecting the jostick grip, A, B, X, Y, RB, RT, LB and LT buttons.
Be careful not to snap off the ball heads when assembling.
Pre-assemble the joystick extension.

Snap in the roll arm to the joystick shaft.

Snap in the roll arm to the pitch arm.

Assemble the shaft and pitch arm with the joystick hinge.

Push in the thunmstick collar and joystick grip to the rest of the pre-assembly.

Pre-assemble shoulder button extensions.
Below the right side assembly.

Align the RB and RT levers accordingly to the bracket and push in the hinge pin.

Connect the RB and RT buttons. Repeat for the left side assembly.

Pre-assemble the action button extension.

Connect the levers to the pushers.

Check the correct order of the A, B, X, Y levers.

Thread B and Y levers through the action button bracket.

Snap on the levers on the action button hinge according their order.
Thread A and X levers through the action button bracket.
Push on the A, B, X and Y buttons to the intended notches on the lever.

Finishing assembly.
Connect right and left base plates and snap on the right and left controller mounts.

Snap on the XBox One controller to the mounts.

Connect the pre-assembled joystick extensions to the base plate. Push the collars onto the thumbsticks.

Connect the pre-assembled shoulder button extensions to the base plates.

Connect the pre-assembled action button extension to the right base plate.

Update 7th January 2022:
I discovered few design flaws and updated the parts:
The author remixed this model.
Remixed the original and replaced the hotas joystick.