The best way to insulate an urban planter box for winter is to protect the whole root zone, not just the soil surface. Start by making sure the planter drains freely. Roots are often killed by cold, soggy soil rather than cold air alone, so clear blocked drainage holes and raise the box slightly off freezing concrete or paving with wooden blocks or pot feet. If the planter is movable, move it to a sheltered spot against a wall, under an overhang, or into a protected corner away from icy wind. Grouping containers together also helps them share warmth.
For the planter itself, insulate the sides as well as the top. Wrap the outer sides with bubble wrap, foam board, or several layers of burlap, leaving the drainage holes and top open so moisture can escape. A layer of straw, shredded leaves, bark mulch, or evergreen boughs over the soil will buffer temperature swings and reduce deep freezing. Do not pack the mulch tightly against stems, and keep it a little away from the crown to avoid rot.
Water plants before the first hard freeze, but reduce watering once growth stops. Do not fertilize in late autumn, because soft new growth is more likely to be damaged. In very cold or exposed urban sites, a simple insulated collar around the planter and a windbreak made from burlap can make a big difference. Check the planter during mild spells and water sparingly if the soil is dry, but never let water sit in the box. The goal is stable, well-drained, gently insulated roots, not a sealed, wet container.