we use a bog box.
I really like it as it means the dogs cant just jump out when the doors opened so if we had an accident and someone opened the back of the car they wouldn't be able to run out spooked and into the road.
Its also good as its plastic sided so they cant really shake mud all over the car. 
That is my fear so he was still attached to his seatbelt which did restrict his movement a bit which i think is another reason why he didn't like the boot. I didn't just want him rattling around.
I woulder if i make a few small trips with him in the boot (after buying a longer seatbelt!) with kongs and a few treats would get him used to it? It would be so much easier.
I don't think the dog bags would help, maybe if we had used these from he being small, he hates not being able to look out the window and i think being restrcited would almost kill him!!
Off to look for a longer seatbelt and some perserverence skills for me now!
L&O x
PS putting him the roof box would probably much quieter for me!!!