We brought our cavelier puppy home in a cat carrier and he’s been travelling in it in the front footwell since, he’s now 15 weeks and getting a bit big for it. We have two high back booster child car seats in the back but at a push he could go on a car harness in between them, I worry something in the boot wouldn’t be secure and take over all the boot space. But don’t know what’s the best/safest to do. Any tips please?