I came across this 35 gallon pond liner at homedepot today. It's 38 inches across at the top (exterior) and 31 inches across at the interior midpoint. At $30 it seems like a great enclosure option.
Patrick
http://www.homedepot.com/buy/beckett-35-gal-plastic-patio-pond-pp1035.html#.UH3FgMXA98s
Patrick
http://www.homedepot.com/buy/beckett-35-gal-plastic-patio-pond-pp1035.html#.UH3FgMXA98s