New version is available http://cults3d.com/en/game/ergonomic-grip-for-3ds-v2-original-3ds
As an aside; whilst I don't have any personal interest in altering the grip for other versions of the 3DS/DS, it's not out of the question if there's enough interest, so do let me know.
The initial version of my ergonomic grip for the original 3DS, same as the one I posted on thingiverse https://www.thingiverse.com/thing:2284483. I've continued to iterate on the original since I first uploaded it and I do have an updated version in the works that addresses several shortcomings but that one wont be available for free.
Yet another ergonomic grip for the least popular iteration of a handheld... I'm really quite happy with this one, it's much simpler and more elegant than the one I made for Vita. No retention peace or anything like that, just plain old friction to hold it in place. The only thing covered-up is the SD-slot but that really shouldn't bother anyone.
I used superglue to put the peaces together and some adhesive pads for a bit of extra friction. I highly recommend water sanding with (at least) grits 120 and 240.
Printed in PLA with an Ultimaker 2+ with a layer thickness of 220 microns. 3 shells (1.2mm) seems appropriate, resulting in a strong print with plenty of margin for post-process sanding.
I prefer printing these sorts of shapes with a concentric top pattern, although be forewarned that the flat (roof) bit on the bottom peace requires quite a bit of infill for the top layers to turn out well.
I'm an HCI student with a passion for digital manufacturing, and I have a serious beef with the all too often poor ergonomics of consumer electronics.