Depending on the size of your box, you can by a pre-built one.
Owens makes one that goes on top of the box.
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CoonDawgs.com - A Site Designed for Coon Hunters
American Houndsmen - Vintage Hound Hunting
Coon Dog Cemetery - Key Underwood Coon Dog Memorial Graveyard
HuntingBassets.com - A site dedicated to hunting with Basset Hounds