Currently, Nintendo's second slide pad solution is the 3DS Circle Pad Pro. One individual thought they could do one better. The result? This.
In order to play Monster Hunter, this 2ch forum member modded a 3DS with a PSP thumb pad, giving the Nintendo handheld dual thumb controllers. To make the whole contraption work, the USB powered thumbpad is affixed to an add-on 3DS grip.
The result is utterly hideous.
No doubt Nintendo will do a future 3DS hardware revision with an added thumbpad. And no doubt it will look better than this.