Foam sled pad (Ukraine). It fits most sleds and is attached with four elastic ties.
The pad makes the seat softer and warmer: it is more comfortable for the child to sit, and the sled feels less hard on bumps. If the pad gets wet, dry it so the foam does not pack down. It is best to store the sled in a dry place: dampness and frost wear out the runners, fasteners and seat faster.
