Hold your shielding sandwich with the magnet on top. Raise the paper clips one at a time to the bottom side and notice what happens. The paper clips should be attracted to the magnet and so will hang from the bottom of the cardboard sandwich. If this doesn't happen, the magnet may be too weak, the cardboard too thick, or the dowel diameter too large. For a quick remedy, add another magnet to the first one.
As you add more paper clips, notice what happens. If you add them carefully, they will arrange themselves so that they are evenly spaced.
Insert the craft stick or plastic straw into the shielding sandwich, move it around, and notice what happens. The paper clips should be unaffected.
Now insert the metal packing strap (or steel butter knife) into the shielding sandwich, move it from side to side, and notice what happens. Do the paper clips fall off? Try experimenting with different materials, such as various metallic coins. Make a list of the materials you test and note what happens.