OP, you're basically asking where we hold our lifetime savings ? Ok, I must admit that place where I keep it isn't safest one. But there is one very important point - keep your cold wallet and recovery seed separately. Keeping it in the same place wouldn't be smartest idea.
And one idea which wasn't mention yet come to my mind while making this post. You can keep your backup phrase buried in your backyard. Use waterproof box or simple bottle or jar. But still, it would be best to use laminated paper in case if something will go wrong.
For me it looks like really bad idea. People tend to forget even more simple things like passwords, phone numbers, PIN codes. While recovery seed basically is just 12 random words - it's not something what is easy to remember. Do you really want to see that you will be able to remember all words in correct order after not using your wallet for year or two? Personally, I wouldn't risk.