I needed to fill in some odd shaped window spaces between log and conventional framing last weekend so decided to go with the 10 mil crystal clear vinyl. It is available in most hardware stores and can be used for windows, storm windows, cold frames or any other place you want a clear window. It is not quite as smooth as glass but very close if carefully stretched when installed. I used thin strips of wood and staples to fasten it but there are commercial plastic strips to snap it in. Here is a link to the strips. It is relatively cheap at $1.95lf here, at 4 feet wide.
Glenn
http://doitbest.com/shop/product.asp?mscssid=93V3X62C1B9A9M8FK3AULCGE1JSU8MK8&mbrid=6680&dept%5Fid=2240&sku=282901