In reply to S i'm on & on:
google image search "car name" + "internal boot dimensions" and review?
If you want it to fit length ways across the width of the car your best bet is one of the van-derived cars, Berlingo, Doblo, Tourneo connect, Caddy life etc. as they're all desgined to carry a euro pallet in their van form so have 1200mm min between the arches. All the passenger versions are cars not vans so aren't subject to commerical vehicle clean air charges but might be charged on age.
Nothing is likely to beat the Octavia for combination of practicality and MPG though.
I'd look in to a custom sized roof box to take the pad (or 2 pads) when opened out, so it can just be transferred between cars in the future and easily removed when not required.
Or some form of towbar mounted box/rack to reduce drag/wind noise from roof box, but then it needs to be folded so shouldn't be kept in there long term.