There was just another thread about this a couple of weeks ago, with a link to another forum! They were talking about makeing sleds out of 1/4" plastic, they had plans and dimentions and all!
Here are two links Outdoors 15 Cu. Ct. Sled With Hitch, Atv, Rmi at Sportsman's Guide Discount Hunting Gear, Discount Hunting Boots, Discount Shoes, Discount Ammunition, Discount Ammo, Discount Boots, Military Surplus, Outdoor Gear At the Sportsman's Guide Cabelas will have them also. Google otter sled or fish trap sled also.