Or you could just throw it on the ice and let it suffocate?
Good for you for letting it go. Although, my guess is that if it is ever caught again it will not swim away.
That one comes with the kit. You could use a square one but you would have to ensure it is pointing straight down. Wayne suggested ataching it to a piece of wood with a cross piece on the ice so it doesn't fall in.
I think it was around $130. It comes with a transducer bracket that holds the cable over the hole but I didn't like it so I made the float using pipe-foam.