I loved the feel of the grips by zeroordie881, however there were some annoyances; There were sharp edges my fingers would dig into when wrapping around the grips, the Gameboy would fall out of the case when opening up the lid or playing in bed, and while in the case you couldn't access the cartridges to change games. Here's what I changed:
First off the pins at the top that click into the wrist strap holes didn't print with my 0.4mm nozzle. Seeing that they were only half height I extended them to ensure they would print.
I also added tabs at the lower corners near the grip. This ensures it doesn't pop out of the case when opening the lid, and in combination of the pins at the top, help hold the 3DS securely in the case.
I contoured the back to add comfort and strength. This thing feels great.
Print just like zeroordie881's design standing up on the bottom of the grips. major supports needed with minor cleanup
If you have the original 3DS XL you can find the grip I created for that here: