This is a topic I'm very familiar with as I think I've tried everything. At first I put it in front of the passenger's seat (since it's just me). Out of the way, but a pain to scoop the poops. Then I put a very large pan in the shower. That works great... as long as you don't want to use the shower. For a while I was using the park showers, but got tired of that.
So... then I bought this cat litter cabinet and stuck it between the seats right behind the hump. This is an actual piece of furniture that actually looks nice. This was great except for two things: I couldn't put a very big litter in it and as a result one of the kitties kept peeing over the edge. The cabinet wasn't water tight so the pee leaked out. Then I got clever and lined the whole thing with adhesive Contact shelf paper. However, I still had to remove the litter and clean up pee from the cabinet on a regular basis. That got old.
My current solution which seems to be working well though which is not as attractive as the cabinet was is an extra large litter with a high dome lid. Keeps the pee in. I put it right behind the driver's seat for now. I'm fanatical about keeping it scooped out, so I also never notice a smell. I do have to vacuum up litter regularly, but I bought a good cordless stick vac and it's not that big a deal.
I'll be interested to see others' solutions.
FR3 30DS 2014