Depends if the phone is supposed to still work.
If you just take an empty shell it shouldn't be hard at all to put an USB killer inside.
If you want to keep a functional phone, that's very difficult. You'd likely have to reduce the size of the battery by half to free some space and then design a custom USB killer to fit into that space. And then you need some kind of switch to switch between regular USB usage and USB killer mode. Or you just hard-wire it to USB killer only and charge your phone exclusively with wireless charging.
Would this be difficult to actually put together? Asking as a mostly casual user of technology and of course for purely academic purposes.
save