6/19/2016: I can't believe I forgot a port for the power plug. Thanks to Tonelock for pointing it out. So...I added a hole, and also trimmed the retaining clips because I found it very trying to remove the 3DS from the grip. Should be less reason to do so now, but still.
This is a remix of zeroordie881's New 3DS XL grip (http://www.thingiverse.com/thing:1269106). A beautifully done model, but it was just too big when I printed it in PLA (possibly due to the ABS vs PLA shrinkage, or maybe it's just me.)
At any rate, as I needed to resize it, I just went ahead and customized it to my tastes, adding the game cart slot, changing the size and positions of some of the ports, etc. I also added retaining clip/ridges so the 3DS would be more or less secure.
It's not as pretty as I'd prefer because it needed extensive cleanup from the supports (I printed with the bulbous handles down like the original) and probably because of the inferior Hatchbox filament I was using just because I already had it. If I was going to do another I'd probably try printing it in "in use" orientation and see how that goes with the support just on the bottom.
Seemed to my paranoid mind (after 45 hours printing) like something might snap when inserting the game unit, but it all worked great (I slid it in from the back until it snapped into place.) Now all I have to do is buff it, or probably not.
You could undoubtedly use a much more sparse infill, I just went for a compromise between strength and filament use that pleased me. I do like the silkiness of the fine resolution, though.