You can use scrap plastic anything to mount them.
I'm not sure about the straw idea or bic pen idea, since the idea of the mounting feet is prevent any metal in contact with the board from grounding out to whatever you've fastened to.... if I understand it correctly.
I used plastic plugs for the ends of the metal tube frames in my ikea desk. I put a screw through the hole in the board, into the plastic, then screwed the piece of plastic to the plywood (in my prototype CP) so that the scew in contact with the circuit board only touches plastic.
Am I correct in that you screw the circuit board to the tube part of the mounting feet with one screw and then mount the foot with a separate screw in the offset foot part? thereby a similar situation where the screw through the circuit board isn't touching anything but plastic?