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.
This is an iteration on my previous grip for the original 3DS (http://cults3d.com/en/game/ergonomic-grip-for-3ds-original-3ds). It addresses some shortcomings and whilst it isn't a complete redesign like my upcoming vita grip, I still think it's a worthwhile improvement. The old version will still be available for free as I think it would be silly to remove it, but know that this one is superior if you'd like to support my ergonomics endeavours ;)
Playing games that favour the d-pad over the analog-pad, I realised that it becomes awkward having the d-pad positioned so far down on the grip. Shifting the console further up goes a long way to amend this but since both pads are vertically aligned there isn't really any optimum in-between and the player will be forced to either shift their grip or strain their thumb whenever they go from one to the other.
Whilst not the most elegant my solution was to add an optional spacer-piece that shifts the console further up when playing d-pad heavy games. This does make the triggers a bit less convenient to reach but there isn't really any way to avoid that without stretching the shape of the grip, and unless you have really big hands this will only puts more strain on your hands and fingers in general (my hands are fairly medium). The headphone-jack is still accessible with the space in place and I added a convenient stylus holder for the sake of it ;)
To accommodate the console being shifted further up I tweaked the cut-outs for the volume and wi-fi switches and I also decreased some margins to provide a tighter fit. There's still some play to allow for padding as plastic on plastic doesn't provide the friction required. For the sides I use a couple of layers of masking tape which works really well (avoid the cheap stuff) and for the retention plates I cut out some adhesive padding.
As with the previous version, it's made with water-sanding in mind and whilst I haven't tried painting it yet there should be enough margin on the inside for that as well. I used grits 120, 200, 400 and 800 for the one in the pictures, although 800 ended up being overkill for my own taste.
Printed with an Ultimaker 2+ in PLA
Layer thickness: I used 0.1mm for the sake of not having to work so hard on sanding down the ring pattern, but 0.2mm will give good results.
Bottom/Top layers: Up to your own judgement but I recommend adding layers as needed for the sake of avoiding a gap in the flat based on the top piece. I also recommend printing the bottom piece with concentric top pattern as this will lead to better results when sanding.
Wall thickness: I tend to print with 4 shells (0.4mm nozzle width), but 3 shells is fine depending on how low you want the resolution.
Infill: Again, use your own judgement but I recommend something like 10% grid pattern in the bottom piece for the sake providing support for the flat roof section.
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.