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PVC makes really nice, smooth boxes/rails and isn't too expensive. mount it onto any width plank from the bottom up so the pipe is still smooth on the top. From there you can just attach supports to the bottom, it's really pretty simple. Make sure you have wide enough feet on the bottom or it will tip over. As far as ski surface in the summer goes fake grass is really cheap from any hardware store and slides well if you spray it down with water and or soap.