The foamboard is regular insulation board (is that polysterol??? I can never remember the name!).
Yes, I did have second thoughts about the strength, especially as the river would be the wakest part of the board and going right through the middle. So rather than using only one board as the basis I glued a secon one underneath, making the whole base about 40mm thick.
I then glued and nailed a wooden frame along the edges of the board. I really took my time making sure that lots of glue was dribbled into any cracks and holes I could see or imagine.
The rocks were given a wrapping of tape on the back and painted with a mixture of pva glue and brown color for a little extra strength (those are the brown areas you can still see on the photos).
Apart from that I tried to keep the weight of the edges down by using lots of water on any glue mixtures. As I worked outside it dried fairly quickly, allowing me to sculpt the rocks. Thanks to the warm and sunny weather everything dried very quickly so I didn't have to wait for days between the individual steps.