Lettuce Raft Concerns

[Swainer]

I'm looking to make a raft system to grow lettuce, but I have worries about using XPS foam and plastic for the bath. Does anyone know of the leaching characteristics of the foam and plastic? Are there alternative, more natural, materials I can use beside foam and plastic. My chemist uncle has made me painfully aware of the negative effects plastic water bottles have and I just don't want the same to be true of my plants.