I need to cover the "shelter group" in my 72 hours evacuation and shelter-in place kits.

I quickly browsed through that pdf tarp primer on equipped.org website and noticed that most designs use a 1:1 ratio (square) tarp.

The closest I found around here is World Famous 8' × 10' poly tarps for only 5 bucks at LeBaron store (I would use one to practice and another brand new, never unfolded would go in the inner sleeve of my Bundeswehr alpine rucksack [my BOB]).

So should I go with the cheap blue rip stop polyethylene type with limited UV resistance and flame retardancy or find a canvas cotton duck tarp?

What is the reason behind the 5:6 ratio (8' × 10' and 10' × 12') instead of square most poly tarps at hardware and camping stores come in?

Thanks
Frankie