Open boot, climb in and press the central locking button.
Climb out of boot and close boot.
Unsure if the boot will be locked or not, tho both door shall be locked. You could try to frig the boot sensor so that it thinks its shut so it locks it, tho then you have the hassle of getting it open again.
Id go with leaving the boot option open, most people wont check the boot. Leave the parcel shelf out to show that nothing is in the boot, and empty the car. Nothing you can do till tomorrow....or the days after etc.
Could strip the door interior out, get to a scrap yard and buy a lock for the door with key if possible, fit lock and lock car via pressing the central locking button option.
Good luck.........