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Out of interest do the Orvis ones have a slit to allow you to access the seatbelt fastener? Couldn't see any mention on their website and that is certainly necessary for our Dalmatian and his Ezydog harness (we just have a cheap and cheerful hammock from Amazon).
Edit: Just had another look on their website and looks like they do have the seatbelt access slits - not sure why I missed that when I was looking previously!