My PC is running back up right now. I'll post the signed message after it finishes and I figure out how to do it. If you feel you need to report this post, by all means, do what you feel need to do
If you don't know how to sign messages maybe this can help you.
How to sign a message?!It's regrettable to lose 11 BTC that's a huge amount we thought, but why not choose a cold wallet as suggested by others as it's not worth such a huge asset but keeping it in a hot wallet which might be vulnerable to hacker attacks when you experience its negligence.
It's just a lesson because it's impossible to get coins back, so in the future, be more careful.
Hope you don't get depressed.
Sorry to say but even if this person learn how to sign a message is the BTC coming back? Are you guys going to get his BTC back for him? Why are you forcing him to sign a message? So that you can believe that he is indeed the rightful owner? What is next after? Let us send his BTC back to him since he is the rightful owner?
OP, I am glad you learned your lesson, you don't have to keep longing this thread, you shared your pain and people don't still believe, leave the post and move on, new readers will learn something from it or also join the group of people that don't believe.
My advice for you now is to get a hardware wallet, it's not even up to $300 to get a solid one, Trezor, Onekey, Keystone, Tangem, D'cent, and so on, these wallets are badass, good for keeping large amount of crypto, now they are doing discount, black Friday's everywhere, act now to avoid future losses, take heart and be strong.