I'm a bike mechanic who have built many cargo bikes and the rattling is inheritance of tubular cage design. Wrapping electrical tape, gaffer tape won't solve the problem but only makes it harder to assemble (grommet is out of question as the noise comes from the insertion opening and not at the end of the tubing). Since current mounting bolts are a further away from the insertion opening and mounted at 3 o'clock position (front) and 9 o'clock (rear), these 2 bolts only good to prevent horizontal movement therefore we need to add bolts to prevent vertical movement. They have to be at 6 o'clock position and offset from other 2 bolts (closer to insertion opening) ... voila`, no more rattles!
Repeat the above modification to other side of the cage :)
Caveat : the additional mount holes have to be drilled into the railing insert in order to be tapped for vertical bolts. If there's no insert material at desired locations, bolt thru and lock nut is other option but it may not look as elegant (inconspicuous) as original bolts!