A knot, which works well in scoubidou is the square knot.
I have drawn it with four strands but you can use more or fewer strands in the center.

Each time one knot from the left and the next from the right give you a flat result, keep working from one side and it will spiral.
If you run out of outside strands let the inner stands jump and take over.
A key fob or zipper holder can be made by folding your string like this and tying with the outer strands. Have a friend hold the top of the strands in your first try.
A fob in 2 colors is made with 2 strands, with the scoubidou plastic you just hold them besides each other. Make sure there are at least three knots over the two strands together.

When you make this knot in the plastic strands you also finish with the last two knots extra tight. If you do not like the look, or work with different string, keep the last three knots loose, work the strands in and than tighten those knots. This way looks better but is harder to do. If neither works, glue might be the rescue, test for melting string.

Have fun.