Which pack does your pup use?
I only have experience w/ the older REI Adventure Dog pack. As long as I unload each side evenly and keep the nylon straps tight and dry it's usually fine. But he has a lot of energy and only 3 legs. So a lot of running around and at a lopsided gait at that make it a little difficult. I'm thinking of fastening a snap in the strap past the buckle to keep it from slipping even after it's wet.

Yeah, rattlesnakes are fairly prominent down here and even more so in the dry desert areas. I too, see them rarely if at all but have heard some scary stories including $1200+ vet bills and, the worst possible out come.

Kinda scary.