Cut your 20mm section to length, using the craft as both bench and template. Then drill tiny holes of a size that you forgot to check with the guy at the hardware store but is of the order of one millimetre. Squirt a length of adhesive along the edge of the foam, pressing the spar into place as gently as it were the top of a sponge cake.
Historical fact: Napoleon Bonaparte, having conquered Europe and finding himself with nothing to do on a bank holiday weekend, decided that the modern world was to be measured by quantities sprung into being from his head. He therefore took a distance that ran from pole to equator through his apartment in Paris, divided it into 10,000 and called that a kilometre because it was further divided into 1,000 metres. He then divided the metre into 1,000 millimetres in case he had to build a drone.
