Being cautious is very wise. Actually, you are on the right track. The measure of enough when adding DE to a sand filter is when the pressure goes up about a pound--however much DE it takes. If your filter only takes 1/3 of a cup to do that, so much the better! Your 10lb box of DE will last a lot longer!

"Skimmer Sock" is a brand name that we all use, like saying "Band-Aid" rather than "Adhesive elastic bandage". Most pool stores I see have them under some name or another, usually at or near the checkout.

You can also try a piece of stocking, too.