You could make a custom Type Arrest_Start Useable Arrest_Start Item with the Strength line having unusually high number or something. But with enough wiggles he could still break free of it eventually.

Thank you. By any chance do you know of a video on how to do that?

I don't have a video but you can create a new folder somewhere on your computer for the mod.
1. Like name it, "Unbreakablehandcuffs" maybe.
2. Then within it you copy and paste from the Steam\steamapps\common\Unturned\Bundles\Items\Arrest_Starts\Handcuffs_Lock folder.
3. You should now have two files in that new folder you made. English.dat and Handcuffs_Lock.dat.
4. Rename Handcuffs_Lock.dat to Unbreakablehandcuffs.dat.
5. Open Unbreakablehandcuffs.dat with Notepad++ preferably/or notepad.
6. The GUID line just delete it.
7. Change the ID number from 1195 to something else within the range of around 10000 to 60000.
8. Change the Strength value from 128 to 999 or something.
9. Blueprints section you need to change the supply ID from 1195 to whatever number you gave this item.
10. Add this two line to the file:
Master_Bundle_Override core.masterbundle
Bundle_Override_Path Items/Arrest_Starts/Handcuffs_Lock
11. Open English.dat and change the item's name and description to whatever you like.
12. Save the changes, then open Unturned.
13. Click the workshop tab, click on submit.
14. Fill in the Name of the mod (It doesn't affect the item, just a page name for the mod you are about to create.) Location of the new folder you create with the two files, Location of a picture to upload as a mod page picture. Category of the mod change to Item, then sub category to whatever you liked. Visibility just leave it as public. Then hit the submit button. This will take a bit depending on network speed.
15. You now have created the mod, click on subscribe to your own mod to use it in singleplayer world/ sub to it on the server by adding it to the workshop list to use it on your server.

Oh yea this item will look and behave exactly like the vanilla handcuffs except that no one else can free the person unless you also created another Arrest_End item that corresponds to this new item's ID. He could still wiggle his way out after long enough though. (The strength value)