Do some aquaponics/hydroponics research. I'm pretty sure most of the "floating" set-ups I've seen have no substrate at all, just small holes drilled in foam to get roots in and out of. Best of luck! Looking forward to the result. Been thinking of doing something similar myself.
Rooted 12 pothos in one lid didn't take long at all did the poke a hole with a pencil slipped cutting through styrophoam with about a half a inch sticking out