If you are that worried use something like a carpet blower in a doorway with a welding curtain hanging and weighted to the exhaust to create a positive pressure in the structure. With your windo open there will be no worries.
Ok fellas here's what I got. I don't have any ball bearings but it had good rebound with a 2 pounder opted for a smaller side plate and also have the bar from the forklift carrage which I'm thinking about using as a horn.
Welded the face and center riser together today as I spent most of my time with the portable band saw cutting that's all I had time for before the family BBQ.
I'm not sure about material but I do know they are off of a 10,000 # rated reach truck and look very similar to what Thomas posted. I have two of them.
Great hopefully I can get to my father in laws band saw this weekend.