Circular Shape and insulating design cover the entire hole to prevent free-up down to 25 degree below. Keeps slush and blowing snow out of the hole. Blocks out unnatural light. Spool shaft is pre-lubed with special Sub-Zero lubricant for smooth operation regardless of the temperature. Spool shaft folds directly into the tip-up base for compact storage. Easily stackable inside a 5-6 gallon pail.