Which kinda leads to another question...where you nervous about digging into the foam without a plywood base underneath? I would love to avoid the plywood under the foam, but I'm scared to dig out my river and go through the bottom of the foam board. For a 5x9, how often did you place your feeders?
Don't forget that, if you're putting a frame around it like flyingtiger's you've got that depth to glue extra pieces of foam underneath to allow you to dig deeper. In fact it would be a good idea to add at least another layer even if you don't.