Model Builder Community

I have made some signs that I want to add to buildings but vannot figure out how to get them from advertiser into model builder. What is the trick? I can save the signs and have done so but since the exstension is mb3 model builder cannt open them and they cannot be saved in any otyher format? I am ...

I do not understand how you can build with foam board as the corners at least one on each side will be bare material with no covering. How do you cover those corners?