Easy solution, beyond not losing the detachable collar in the first place:
Buy a secondhand jacket with its detachable storm collar.
Keep the collar and sell the jacket on.
With luck, you might even make a few quid on the way through.
Thinking outside the box; it's the Adventure (with a capital A) way.
See OP, I told you that was the way to do it. After all, it was exactly what the seller of your storm collarless jacket did to you.
Think like a biker, mate and all your problems will just vanish away.

