i haven't routed foam before, but i have cut it on a table-saw, and the foam tends to grab onto the saw blade, and bind up. when the foam is moving past the blade at a good rate, it is fine. but slow down, or stop, and bad things happen. if there is much flex at all, i can envision the router bit orbiting around in the foam. others with more foam routing experience may know better. i just wanted to voice my concerns.
i understand the whole low-budget build. before i just happened to catch the Craigslist advertisement, for my router table, i too was looking to use inexpensive materials. seemed like every design i came up with, i was told to build it stiffer and stronger. i had even setup an experiment, and tried to route a simple sign using a wood router, completely free-hand. the forces of the bit cutting into the material, and wanting to take off on it's own was surprising.
also, on the note of cutting foam. i know when i cut it on a table-saw, the foam "dust" gets absolutely everywhere, and sticks to everything. i would suggest thinking about some sort of dust collection. even a shop-vacuum should work, until it fills up.
not trying to discourage you and your machine, just trying to pass on what
little experience i have.
~Travis