@henshaw49What type of dog pack are you using?  Do you know the brand?  I have two ideas you may want to try.  One, contact the company and see if they have any extender type clips that work specifically with their products.  It's worth a try to have something that will work easily with your existing pack.  Second, what if you were to give your dog a shave around the waist area to gain a couple extra inches?  I wouldn't shave anywhere the straps would rub like under the armpits, but just around the waist it might work.  Plus it'll give Fido a cool summer do as well. 

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