Head to your local hardware store and look for a Hitch Pin in the size you require. My favorite for small sleighs is the hitch pin with the c-shaped wire retainer and a short piece of wire tying the wire retainer to my towbar so it can't fall in the snow.
Larger towbars for hardside huts can use a bigger diameter hitch pin, like those found on farm equipment. They've usually got a really high breaking strength to tow heavy farm implements, so a fish hut shouldn't be an issue.