For testing, use bowling ball sized rocks, that are not near round, so you can move them easily in small increments.
Wrap the line all the way around the rock then around to the tie off, use a slip knot, that way the line grips the rock better.
I like using rocks since they will move a little, if a really hard gust of wind grabs the tarp. I would rather have to re-position a rock than to have a torn grommet from the strong tie-out line that we have these days.
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