I got the Promag 9mm magblock and right off the bat I'm not sure I like it. Not because it's body is made of plastic but because it's a drop-in from the top and it has a bolt-hold-open feature that requires that the bolt hold open on the AR lower must be temporarily removed to allow the magblock to be slid into place. Then the bolt hold open on the lower must be put back together. The only 9mm Colt type magazine I had was a Promag plastic one and it seemed to work just fine but I was hand cycling the ammo. I did have some unmodified uzi mags that I slid into the lower with the magblock and they fit ok but, along with the Promag 9mm magazine, they are not drop-free. The picture below shows the magblock in a lower with the lower's bolt hold open removed.
The feed ramp seems to be steel and so is the ejector hook, the rest is some type of sturdy plastic. Notice that I bent the ejector hook a hair to make sure it fit into the slot on the bolt just right. I don't know if the magblock can be modified to allow it to fit in without removing anything from the lower. I'm going to hang onto this magblock and maybe I'll build a cheap dedicated 9mm lower using one of those "Plum Crazy" lowers.
This is the package the magblock came in.