The reason SLO, and Flippac rain covers sag so much when wet is because they are probably made of nylon. Most rain covers for higher end tents are made from polyester for this reason. Polyester fabrics absorb much less water and therefore stretch at a greatly reduced rate compared with nylon. There are advantages and disadvantages to each material, however, in most cases polyester is the better material for rain covers.
Cliff
Cliff