What saddle rail clamp should I get?
Those style of saddle clamp only slip like you described if they’re loose. Make sure you tighten the side bolts really good (it’s hard to break those things) and use threadlocker.
A better option is to buy a 2-bolt (also known as “infinite adjust”) seatpost. Those use a better design where that slippage is not possible. Simple aluminum posts are not too expensive. Make sure you get the right diameter and length for your bike.