I've found that making 3 knots tends to help especially if you choose not to use an adhesive on the cord. Knot once, using any strong double lace knot, where the 2 cords meet then pass each end back through the beads to the left & right of the 1st knot. Then make any type of locking single lace knot after those beads, feed the elastic ends through one more bead on each side before snipping off the extra. Never clip any cording matterial flush to a knot, always leave a "tail" of at least 1/4 of an inch (about 6mm), this way if the knot slips there is some more stringing material for the knot to grip onto.