Since you want it to be longer than the string can be, you'd have to compile the hax payload from the source code. If you're using a hex editor, it can only be nine characters total (boot.3dsx is 9 characters, so you could have a 9 character file with no extension, emuBootLD for example). I think most people just use something like emun.3dsx (emunand) or emub.3dsx (emuboot?). If you go past 9 characters with no extension, or more than 4 chars with .3dsx as the extension, you'll break the payload.