Tubb's my big old staff who used to ride on the Beemer trike, used to be fine on the bench seat although I never went that fast with him on board 50 mph max. But when I used to go over about 40 he would move across the bench and sit directly behind me. I only ever used to clip his harness to the seat belt enough so that he could move across when he wanted to.
Little Deef is a Jack Russell &I haven't adapted the tank bag yet for him as he's still only a pup and I'm not sure how big he'll get, although I know you can buy "Doggles" for your dog and have seen all manner of adapted topboxes and bags for dogs on bikes and trikes. Most are homemade but some are commercially available.
Some clever ones are a topbox with a perspex bubble and air holes at the back, another with an internal harness a hold in the top with an adapted double bubble screen in front. my favourite is a guy out Ipswich way who has a trike with a lined box on the back with a lid on telescopic rods on all four corners so that his staff can pop the lid up about 6-8 inches when he wants to have a sniff.