Leather seats and car seats

Today I put the base in my husbands truck that has leather seats. Even with it as tight it will go, the base wiggles around a bit more than I’d like. Is there something I can put under it that is SAFE to keep it from wiggling like that?

Also, can I or am I supposed to use the LATCH system and the seatbelt?? If not, why?

I also plan to take it to be checked by a local CPST so nobody get cranky about my questions!

I have read the car seat manual and the vehicle manual on the latch systems, where is safe, etc. If it had answered these questions, I wouldn’t be asking!