Hi Rockys girl and welcome to the forum
I have a wee yorkie so just tie his harness to one of the back seat belts on back seat, and do not need a dog guard or cage, he would bark his head off if put in the boot, as does not like to be alone in car.
Plenty choices available on-line or car parts shops like Halfords. Most are ugly metal things.
if you can do without a cage and just let the dog free or tied in in the boot then the best looking dog guard out there is a perspex one, that make viewing out the back a lot clearer.
perspex guard...
metal grid guard...
dog cage...Not as simple to pick as it depends on dog size and dimensions of boot, here is one example of different sizes.
Good luck, someone else on here might recommend a dog cage, but would need to know your dogs breed/ size.