At dawn, I sneak to body-double-san's bedroom and leave the replica on the bedside.
I put a note that says, "I've prepared the replica, please use it as you see fit.", together with the replica. With this, even the nitpicking nobles of pedigree can probably be effectively deceived. If the kingdom protector holy sword went missing, they would find someone to blame even if the duke and the king had given their permissions.
However, I'd have preferred a beautiful woman's bedroom if I had to sneak in anyway.