I am NOT a fan of the zdus at all. They are famous for popping up(if not mounted perfectly) ...and then you can lose the part.

YES on the filling. ALSO ...with the metal tubing ....that offers a TON more stiffness. The foam offers NOTHING but a form for the fiberglass mat bracing. THIS is OK .. but if you are going through all this ...do-it with metal. WELDING IT would be better but not really required.

I have done the metal bracing MANY times in regular production. The trickest ? ....magnesium tube.