Maybe a metal footlocker secured to the seat mounts?
we visited costa rica, driving a small, rented suv.
we left our backpack-sized luggage in the back, uncovered, for all to see, with one of these secured to the car seat frame.
? pricey, but secure.
go
here no problems. figured that if someone wanted it bad enough they were going to have to really fight for it - and that there were other, more convenient targets.
they come in numerous styles and sizes.
good luck.