A 3d printable 3ds ar Card if you'd like something more sturdy than the paper one the 3ds came with. I have made 2 versions one is printed in whole and needs to be painted to work and, the other is printed in separate pieces if you don't want to paint it.

I printed the one you see in the picture at a scale of 450% but, you can probably print it smaller at 200% or 300% and it should still work. Printing it smaller may prevent the 3ds from recognizing it.

!a helpful tip! if you plan on painting it is to use a black paint marker or something similar as the paint won't leak into the Nintendo logo.