Car/booster seat recommendations.

I’ve got an NC2 touring (yes the airbag is disarmed, it’s the main reason I bought an nc2 over an nc3 was for the airbag disarm switch rather than weight sensors.) and my 5 y/o loves going on rides. It’s getting a bit old moving the car seat from the minivan to the Miata, and he is right at the weight for a high back booster. And recommendations? Also do you just put in the car seat or do you recommend a car seat pad under to protect the seat?