Renters doorlock (53mm)

Being a renter it's not always possible to secure all doors.
The last place i lived in had 3 doors, 1 of which was a prime candidate for a smash and entry.
Although i eventually put a padlock on it, this was just a super simple way to prevent the door from being opened.

I printed 2 of these out and put a M3 bolt through each hole, it was just loose enough to spin around but tough enough that you wouldn't be able to break it.
It was not possible to unlock the door without tools. certainly enough to put my mind at ease about an "easy" door to break in through.

Yep, they (burglar) could jump through the window but hey, while the alarm system is screeching and security on their way their time is limited and a non-cooperative door is just what is needed to slow them down :)