A few of you (those I gave a mention to on the Facebook post) correctly supposed that the best approach was to have two piles of five rice cakes each, and then place the other four, two in each of the spaces left over.

The surprising detail that no-one managed to provide, is that technically they don't fit! There is a very slight overlap - a fraction of a millimetre - at the four points shown in the diagram below. Had the rice cakes been just 1.9cm thick, there would have been no problem (and in fact real Snack-a-Jacks are slightly shy of 2cm thick anyway).

For any practical purposes they do fit, which is why I was happy to accept that answer if the arrangement was optimal.