Update 18/10/05:
I've now uploaded the newer version on Thingiverse as well: https://www.thingiverse.com/thing:2970246

Update 17/11/03:
Since it has come up in the comments I'm going to use that as an excuse to shamelessly promote my grip for the Original 3DS Xl
https://cults3d.com/en/3d-model/game/ergonomic-grip-for-the-original-3ds-xl

Update 17/07/17:
New version is now available: http://cults3d.com/en/game/ergonomic-grip-for-3ds-v2-original-3ds. This version will remain available for free on Thingiverse but if you'd like to support my endeavours then consider buying the new version.

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.

Sidenote 09/06/17:
With some minor sanding this grip will fit a DS Lite.

Original text:
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.

Ultimaker

Ultimaker 2

No

No

220 microns

10% Zig Zag, 1 step granular infill (bottom peace)

Printing with PLA, 3 shells (1.2mm) seems appropriate, resulting in a strong print with plenty of margin for post-process sanding.
I like printing these sorts of shapes with concentric top bottom patterns, although be forewarned that the flat (roof) bit on the bottom peace requires quite a bit of infill for concentric to turn out well and the more top layers the better.