I might be concerned about any uncovered parts being either too hot in summer (in direct sunlight?) or too cold in winter. Using fleece and towels and papers would insulate the base to some extent - but you would have to pull the fleece up and over the edges to cover them.
Thank you all - good points. We are adopting and I'm looking for something rather quickly to get her home as soon as possible. We have lots of the sheet metal available. It is pretty lightweight but it may be too cold for her. I'll keep looking for the colorplast too.