To do this, solar pool cover will be cut into pieces and tied either to a PVC framework around the cart, or directly to the metal rungs. This cover is usually meant to conserve pool heat, but has been found to keep greenhouses warmer by 20-50 degrees during winter (see this discussion). It's essentially bubble wrap, except a lot sturdier.
A heater was purchased, and it will be connected via an outdoor-rated power cable, preferably one of those heavy-duty 14- or 12-gauge ones, with a rating of 1800 watts (since the heater draws 1500 watts on high). It will be placed below the lowest shelf, or attached to the side, and hopefully keep the temperature inside at 80-85 degrees. Given that the inside volume is about 60 cubic feet, that shouldn't be a problem.
This will be a great help for starting seeds in early January so that we can have decent 4"-sized plants for the spring plant sales.