You might find it weird, but I keep my mnemonic phrase written in the small list in an empty ledger cartoon box. I have thrown that box into one of the boxes of my cupboard next to bed, together with mobile phone, laptop and other gadget boxes. And guess what, for 3-4 years it has been save. Want to know why? Because that cupboard box is a mess and no one wants to organize things there. Because this is a box graveyard that might be needed one day. Since this is considered as my cupboard, no one ever gets there.

The important thing is even for the phrase to be safe and easy accessed when needed. Its importance i won't keep it there if i was the one, and looking at the duration it has stayed there means its safe for you to keep it there onlike me that anything that involves money I won't be even comfortable i will start thinking of several ways just how to keep it safe, once someone has access to it then thats the beginning of the problem. I love the psychological play using the natura of that box to hide it there no one will even think of going through that box except maybe the person is looking for something.

I have two methods :

I have laminated pieces of paper stored in secret places that are safe against theft and natural disasters. (high and dry and capsuled) AND I encrypted the phrases and split them up and stored them in online storage. (Only people close to me, will be able to decrypt the riddle I encrypted) ....and I left clues to it's location.

We decided to take the responsibility of storing our wealth on our own, so it is our responsibility to keep it safe. (We live in times of war and natural disasters, so we should step up and adapt to the changing times) 

I use a different method that I'm not going to disclose to the public, but I have another idea that might help you. Based on your phrase key (12/24), you can divide them in half and put them together again to make one key, so the middle part comes first, and the first one goes in the middle. That way even if someone gets their hands on the phrase key, they won't be able to access it easily.
Here's another idea. Find a book that has the same thing written on every copy. The book needs to contain every word that comes in the phrase key. One common choice could be a dictionary. Then you can find on which page the words are on and then from top to bottom you can mark the number and the page. After that you can write down the page number and word number in a piece of paper or wood or something metal. That way only you can understand it, and you can decode it in the future.

All the last thing you can do is combine this both and create a much stronger storing system. If one is smart then they might be able to crack it, but that won't be easy. My own method is related to math, but I'm not going to share it. If you think these ideas are good enough, then you can give it a try.