Hey OP,
I bought a couple of those cheap jacks adapters on eBay and was not impressed. A plain old hockey puck works much better at spreading the load on the pinch weld while not damaging it.
Yes, good for a Miata, where the lift point is next to the pinch weld and not on it.
On a lot of vehicles, the pinch weld itself has a reinforced point for lifting. In that case, the pinch itself is the lift point, not the rocker next to it.
You could take a hockey puck or block of wood and just router a channel thorough it.
Would that be safe? I guess the mass of the vehicle and gravity would mean that the block of wood wouldn't shift? Even if it's not bolted to the jack.