Important notes:

• The printed shell will cover the ambient light sensor making it inaccurate. It's best to disable the auto-brightness option after doing the mod.
• One permanent modification to the OEM rear case will be performed with a precise 1.6mm hole to help secure the antenna shell to the console. Alternatively you could ignore the inner screw bracket and use strong double sided adhesive (the type used for phone screens). That will not be as securely permanent though.
• I would recommend having an eyeglass repair kit. One that contains washers and screw nuts. You could use the screws provided in the kit for both inner and outer brackets. The screw driver will be important for securing the inner screw to the Switch's rear case.

Print notes:

• Slicer settings should prioritize precision. I would avoid fuzzy as it may prevent correct fit.
• Used 50% infill.
• Support structures recommended on the inside going vertical. Leave a slight gap for the inner screw bracket. Ideally support structures don't touch the front, top, or back inner walls
• You may need to trim away any unevenness from the PLA settling on the print bed. You could extrude the bottom base of the shell slightly beforehand and slice it off after printing.

Resources used for the mod:

• 4.4mm Phillips - Inner screw
• Screw nut - Used with the inner screw to fasten to the rear case.
• Superglue - Only a light amount to secure the inner screw on the inner bracket to fasten the screw nut.
• Theadlocker (Loctite) - Keeps the screw nut from getting loose on its own. Important to avoid possibility of shorting out components.
• 4.4mm Phillips - The outer screw which will use the same screw hole as the 2mm long screw used for the console's rear case. Recommended same thread pitch length.
• 1 MOLEX 1461530100 2.4-5G antenna - The replacement Bluetooth antenna that adheres to the front of the inner wall of the printed shell. These can be found on Ebay. 
• 1.6mm (or 1/16) drill bit.

Assembly steps:

1. Take the finished printed shell and ensure the fit on the fully assembled console by lining up the outer screw bracket with the 2mm screw on top of the console. The left side of the printed shell should not touch nor cover the "+" volume button.
2. Use the 1 MOLEX antenna and adhere it to the inner shell front wall. The fit will be precise, go slowly. Make sure the antenna touches the top of the inner shell as far as it can go.
3. With the 4.4mm Phillips, place a tiny amount of superglue right under the screw head then quickly place it inside the shell's inner screw bracket and put pressure holding the screw down until the solution solves. Use angled tweezers. The screw should not rotate freely.
4. Disassemble the Switch so the entire rear case is off. Drill the inner screw's 1.6mm hole in the Switch's rear case next to volume and power button rubber pad. This is to securely fasten the left side of the printed shell to the rear case. Careful with the placement so that the outer screw bracket still aligns with the existing hole. If you need to make the hole bigger, make sure the screw nut can't go through it by using a washer. 
5. Remove the ambient light sensor cover on the top of the Switch's rear case. It is right next to the 2mm screw.
6. Feed the new antenna's cable through the ambient light sensor hole in the Switch's rear case.
7. Place the printed antenna shell on the rear case. With the 4.4mm Phillips screw exposed on the inside of the case, apply the threadlocker solution onto the screw.
8. Place the screw nut and fasten it snug to the Switch's rear case using a screw nut driver. 
9. On the console, disconnect the OEM Bluetooth antenna which is located right next to the joycon rail. 
10. Remove the white tape securing the OEM antenna and unscrew the OEM antenna from the shield plate. Fully remove the OEM antenna. Keep one of the pieces of tapes handy.
11. Place the antenna screw back and use a convex washer (often included in eyeglass repair kits) to retain the same tightness on the shield plate.
12. Take the Switch's rear case with the printed shell and attach the new antenna cable connector to the motherboard.
13. Reuse one of the white antenna tapes removed earlier. With the new antenna cable suspended, stick the tape fully on the antenna cable but not to any other surface yet.
14. Place the bottom part of the rear case first so the tabs are snapped in place with the bottom corners and center. Careful not to yank the new antenna connector out of the socket.
15. With the top of the rear case slightly cracked open, use tweezers or a spudger to stick the tape and antenna cable to the shield plate. Make sure the fan is not covered by it.
16. To finish, resume reassembling the rear case.