I agree with your statement about 60 inches might not be wide enough for a 70 inch wide footprint. I would go 65 -68 wide and 88 long for it. I have a Marmot 3 person with similar dimensions and ordered a piece off of Ebay to my exact request. Sorry I don't remember the name but I did a search on ebay for Tyvek and a number of places popped up. Chose one and gave them my dimensions I wanted and a week later I had it. A little less then $15 all in.