Sorry for the late reply.. the expandable foam will dry with a smooth surface, unless you mess with it. It will be flexible, though, so take Erik's advice into account. I'm not sure how flexible materials would effectively increase the internal volume, but Erik doesn't tend to talk out his butt and has much experience building speakers.
That said, nothing wrong with experimenting.
If you do use 2x4's, considering using blue tack to hold them in place until you're happy with the sound. Then, if the cabinets are unpainted MDF on the inside, consider wood glue. It is very strong when gluing wood together (duh) and would not be flexible like silicone, which could add too much damping. Or maybe you need the extra damping of the silicone.. I suppose you could start with silicone and, if it's over damped, cut it out and use wood glue.