I've got one of the shelters installed in the garage slab. Not exactly the same you're looking at, but I did some research before I bought one. Ground Zero is run by idiots. I don't know if they even make an above slab model, but avoid them if they do. I ended up install one built by Flat Safe and I was very happy with their installation.
My slab is post tension. The builder installed a block-out for me in the garage so I could install a shelter at a later date if I wanted to. I met with the sales manager at Ground Zero and gave him the dimensions of my block-out. He said that it would work fine. Install crew showed up and said it was too small and they needed to cut the hole larger. I told them they couldn't go larger or they would cut one of my post tension cables in the slab. They gave me the owner's number and left for their next install. After 4 messages and 2 weeks later, I finally talked to the owner. He said they could try and make it work, but no promises. Then he went on to say "we cut post tension cables all of the time. It is no problem if we need to cut one in your slab." I hung up and called Flat Safe. I can not believe the dumbass said that.