Former Paramedic, ACLS instructor and CPR instructor here. Lots codes in the past, but currently not working in ems. (I'm LE now.)

Back in the day, this was the best thing going because it was much easier to get a good seal. http://www.allmed.net/catalog/item/87/523

There were many times that it wasn't necessarily easy to get a good face mask seal. There may be something better on the market now. Like I said, I'm a little long in the tooth so there may be newer things available.

As for those little fold up face shield things, I carry one but they are less than ideal. You car or house kit should definitely have a real mask. In a real CPR situation, there are blood, excretions and vomit to deal with. You get some on your gloves then you have to hold that face shield to get a good seal and guess where those fluids are? On the outside of the shield. It's better than nothing but not by much.