Don’t glue it shut, paint it shut with an acrylic paint or nail polish. Try to stick to the surface with the paint - you could even paint it to cover it/obscure it’s location as well.

This way, if you ever need to remove it you can delicately dab acetone over it to dissolve the acrylic and it will open once again!

SIM cards do sometimes malfunction, so if that happens and you glued it in you're kinda screwed.

There isn't really a point to that. The very first thing thieves do is turn off the phone so you can't track it. They'll then usually format your phone and sell it. Best you can do is set up a secure password and not show any info on the lock screen so they at least wouldn't be able to access your stuff.

Depending on how they get stolen, I'd carry a decoy.

The federal government has had difficulty dealing with modern locked phones. The sim isn't integral to the security of the device, it's just a pointer for the carrier to route calls to. A modern phone with whole system encryption and a strong password is going to be protected against most everyone including nation state actors.

The only real advisory I like to put out with regards to phones is the same as most other devices, don't reuse passwords and for two factor ideally use something other than email or SMS which makes it far more difficult to get the second factor.

The other thing that's kind of a 'best practice' is using a password rather than any kind of biometric. The reason there is last I'd read (in the USA) the current legal guidance is that while authorities can compelle someone to put a finger on a screen or look at the device, they cannot compelle them to open the contents of their mind (forced testimony) due to the 5th amendment protections. Of course that kind of thing is more related to legal situations rather than lost/stolen devices.

I might recommend using something like tape. Extra Strong Clear Tape.

You may also want to invest in a phone case; preferably an opaque and waterproof one that is difficult to remove.

There are apps available on F-Droid like PrivateLock which might help too; this will automate locking your phone if it's snatched from your hands suddenly.

• PluckLockEX: (No longer maintained) https://f-droid.org/packages/xyz.iridiumion.plucklockex
• PrivateLock: https://f-droid.org/packages/com.wesaphzt.privatelock

Last thing I can say is to use a short screen timeout and a strong password to unlock the phone.

To actually answer your question:
Super glue would be a decent option. You should only need one drop to secure the tray.

If you ever need to open the tray again, wet the area with isopropyl alcohol for a few minutes, and it should pop right open without damage. If you get too much resistance, add more alcohol and wait a little longer.

There is also a 2-part epoxy with similar performance called "Doubkr Bubble" that also dissolves with alcohol, but it's definitely overkill for a SIM tray.

A better option would be to use a good password generally and the built-in lost device functions like Find my iPhone, Google Find my Device or Samsung Find my Mobile.

If you're using Android, and want a separate app that can lock down your device outside of the built-in functionality, among a ton of other features:
I used to use Cerberus years ago for that security before that functionality was really built in. It is a side loaded app that allows remote tracking, lockdown, etc. and functions as a system app. They have a version of the apk that is listed as "System Framework" with a stock Android icon and as well to hide in the app listing. You can have it hide the app and access it via a dialer code as well for added security. When I used it, you could have it take photos to send to your email with location data for incorrect login attempts, send messages to the device to display while lost, etc. www.cerberusapp.com

I mean on average theyll just turn it off and then try to factory reset it later which will remove your data if there isnt an exploit. What i do since I have the same problem, is just not keep any data on my phone the same goes for accounts but I see that might not be for everyone. Assuming your phone is fully updated a long passphrase, think stock android limits to 17 characters, should be enough to keep out the average drug addict. Be mindful of your surroundings, dont fight back unless they try kidnapping you and use lockdown mode or fully restart your device when you go to a shady area. Curious what device do you have?

Edit: what you should be more worried about are the assholes that force you to unlock it and then empty out your bank through the countless digital payment methods. Arming yourself would be the answer for that

Some phones including Samsung are encrypted by default.

Don't keep data on your device you don't need and use the encryption.

Have you thought about using a strong unlock method, so they can not gain access to your data in the first place (if all you want to do is wipe it)?

Thiefs just throw phones into faraday cages to block all access immediately, then they fiddle with it at home. You're pretty much screwed either way.

If I really had such a concern I'd use a Linux phone, e.g PinePhone or PinePhone Pro, and I would program it so that unless a certain command has been ran, if it boots or stay too long (e.g 1s) without a SIM then it deletes sensitive data.

You could just set a Sim pin