Thanks for the recommendations for good seats, I'll have to look into them before my next trip.
Although I have to say I still think that if I'm paying about USD100 to rent a car seat it is reasonable for me to expect that the car seat is good quality, fit for purpose and properly installed. Just like I like to think that things like the brake pads, seat belts etc and all other safety equipment on the car is good quality, fit for purpose and properly installed.