you could install the floor just like it was, some do it like that, most fiberglass the wood in. by pre fab, I mean they sell you an exzact fit peice that fits in your floor section, has the trans section cut out where the tunnel goes ( that is fiberglass, buy 1 from them if u don't have 1. ) you could make whatever you needed, by making your own template and cutting it out, use marine grade plywood, it rots out slower and is what most use, and T buckets pre fab floor is made of it.