Thanks all for your input. I purchased my vinyl yesterday. I found a supplier who had some surplus 72" wide green vinyl used for commercial awnings. This stuff seems quite rugged. The best part is it was only $8.00 per yard.
I have read discussion on this board about skinning the outside with one layer and/or 2 layers of 1/8 luan. I am going with the single layer of 1/8 luan, applied 3 coats of exterior varnish and am going to use contact cement to glue the vinyl to the top. I will get some pictures once it is all in place.
Thanks again for your input, suggestions, ideas and information!