How big is the prop? I can tell you for sure that there are ways to make it work. It seems you are shoving it into the foam, correct? If this is true, Sharpen the dowl with a knife or whatever (I used my pocket knife but there are other ways), then use an exacto knife or other cutting type small blade, and dig a hole into where you want it inserted. Glue inside the hole (most insulation foam props don't need it more then about 5" in), insert and let dry.
This is my proof (it stood up to tons of wind at a con in the fall):