Sorry, poorly worded,
yep i used the ply just to hold the thick(ish) card so it didn't rip when drilling through.
i use the card from stationery supply boxes, it sits between envelopes at fixed points so they don't sink in the box.
it's thicker than a cornflake box card, but thinner than corrugated cardboard.
i did try using plasticard, but card is easier, at least for me.