I don’t know if anyone else is interested in a case to hold your gameshell or not but I today I bought a Nintendo 3DSxl/2DSxl case from Target for $10.00 The color matches my gameshell nicely and it fits perfectly. I used an exacto knife to remove the Nintendo branding and replaced it with one of the awesome stickers that came with the gameshell. I also removed a few stitches on the elastic that was meant to hold DS games and was able to use it to hold one of my micro USB cables. I have not tried putting the game shell in with the lightkey (mines not with me at the moment) but I believe you could fit that in the case as well. Anyways hope you enjoy, Happy hacking. 
edit: you are actually able to store the gameshell with the lightkey in this case. more in the comments.
That’s neat. Is there any hard case for carrying it around with the shoulder buttons attached? I’m really like these extra buttons even if I accidentally broke the plastic between them when assembling the unit. Even separated of each other they work really well and contribute well enough for GBA games.
so I’ve tried now that i have my lightkey with me and you can lay the gameshell face down and still manage to zip the case but its a little uncomfortable in my opinion, if you leave it plugged in but detach it from the back and slide the little brackets off it fits much easier. also you could probably just remove the partition in the middle of the carrying case and i imagine the lightkey wouldn’t be an issue at all or another option is just unplug the cord from the light key, detach it and store it in the front where i have my usb cable.
Also if you go into your local target and look at the cases, there are “deluxe” 3ds cases that are much bigger and there’s also a lot of nice hard cases for the Nintendo switch that might work.
good luck and happy hackinng