A very easy to tie stopper is essentially two overhand knots
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Make a bight and tie first overhand knot
Bring left hand strand over the bight
Bring right hand strand under the bight
Bring the uppermost strand down through the right hand bight
And bring other strand up through its bight
Bring the right hand strand up through the small bight of the first overhand knot
And bring the left hand strand down through the other bight
Grab the loop in one hand and the two strands in the other hand and pull apart. Adjust to tighten