Sizing your dog for a K9 Sport Sack® isn't an exact science. If your dog is in between sizes, has a wide body but short in length, or is still growing, the Booster Block fits snuggly in the bottom of your K9 Sport Sack to boost your dog up 3." It helps make the K9 Sport Sack experience more comfortable for you and your dog.  FEATURES * Constructed of quality foam * Removable and washable cloth cover * Two blocks may be used to lift up to 6" SIZING * XS/S are interchangeable between XS and S sized backpacks * M/L are interchangeable between M and L sized backpacks * XL backpacks require an XL Booster Block * XXL Bags require an XXL Booster Block  

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