Portacrib Mattress

We bought our portacribs girls. Including plenty of room now and will be perfectly portacribs now. They are 7 months.
We need to get a better mattress, then what came with them. The mattress is so thin said.
What is the difference between a block and a mattress portacrib portacrib? I see more blocks, then mattresses. What should I be looking in the pad?
Any help is appreciated, I'm alittle confused

Here are some general guidelines mattress

http://www.essortment.com/family/babybed ...

And I would like to add a protective vinyl on the mattress with a mattress - the mattress to protect BCZ I belive there will be times when the layer does not

A mattress thicker than a pad.