This page shows the process of installing new padding and carpet in our 2000 Honda Odyssey. When we bought it, the existing rear seats and carpet were in pretty bad shape with quite a few soda spills, having previously been owned by a family with small children. We removed the rear bench seat and middle seats. We also removed and discarded the old carpet and padding.
We bought new padding from Lowes. It is meant for under wood floors but seems to work well for our purposes as well. We also bought some marine carpeting from Home Depot. We removed most of the interior panels to clean them on both sides because soda had also gotten behind the panels. We then removed the front seats to give them a thorough cleaning.
You can see from the pictures this was quite a process. When this was finished, Chris and his dad began constructing a bed in the back using metal pipe and plywood for the platform. We used memory foam for padding. The bed sits a little over 2 feet from the floor, allowing storage for our clothes and stuff underneath.