Okay ... after getting some well-needed experienced advice, I have come to something that will make everyone happy, and I will feel comfortable with.
Neo suggested I get
Owens Foamular from Home Depot, which is covered in a plastic film, therefore making it impossible for the foam to eat away at the acrylic. Thanks Neo!
I don't know what everyone else is using, but this will work for my application. If this is what you were talking about, that's fine, but specific details are important
Now ... on to the build. Will be posting plans soon and buying materials soon as well, once design is confirmed.