[Screwneck]

No, turning the magnet around makes no difference whatsoever. That's like saying if you turn a chocolate biscuit upside down then it tastes better

You can buy thin magnets on Ebay and then fill the gap with strong cardboard or styrene as a shim. Make sure you buy magnets of the correct measurement as most Ebay sellers do not fit Scalextric cars even though they say they do.