You might want to consider substituting the rolling hitch for the clove hitch. It's basically just a clove with one more loop under the overlapping piece, but it's more secure.
The load on a clove hitch should be perpendicular to the pole (or standing piece). The load on a rolling hitch should be parallel (or a very acute angle) to the pole. In either case, if the direction of the load is wrong, the knot can slip. Choose the knot that best fits your purpose.
Regards,
Eric
