For an idea, see the semi-hardmods link in my signature for the section Extended Power Button Mod.
If this is for the o3DSXL, I recommend disassembling the 3DS to get to and remove (POWER) button that's already there, sanding down flat the top surface of that button, buying another o3DSXL (POWER) button, sanding the bottom surface of that replacement, and then super glue the 2nd button on top of the 1st button to raise height by 2 mm.
You don't want to super glue the buttons without disassemble in case you accidentally glue the buttons shut.