you might throw in a dozen or so furring strips to attach the tarp edges so wind and rain doesn't get under it....a can of expanding foam in case of a puncture...

for those who don't know what a furring strip is

http://www.homedepot.com/p/1-x-3-x-8-Furring-Strip-Board-164704/100088011#.Ukc6tEfD8qQ