IMO your problem stems from the ‘feet’ on your plastic containers and the 8 mm gap that produces. The effect of this is the cord has to heat the air first and then the warmed air has to carry that heat up to the base of the container. Problem is, warm air rises. So what I would say is happening is that a convection current of warm air is establishing, taking heat out and upwards at the back of the containers and drawing cooler air in from the front and sides. You would likely be able to feel this with your hand once the heat cord has been on for a while. Alternatively, use the smoke from an incense stick to check it out. This is where I reckon you are losing most of your heat. To fix it, you need to eliminate this air movement. This is why it was recommended that the heat cord should to be in contact with the base of the containers (Kaiwei, Herptology, rupertbernoulli).
A cheap and easy way to solve this problem is to use several thicknesses cardboard (from boxes and glued together) or 8 mm thick Styrofoam sheeting, to hold the heat cord up against the base of the containers. Use strips of about 10 to 15 mm wide and just long enough to allow for the back feet on the containers to pass either side of them. The heat cord between each strip will also need to be taped down to let the back feet pass over them. Partially recess the cords into the cardboard or Styrofoam so they just sit a little proud, and hold them in place with aluminium tape as suggested by Herptology. Try this on just one container first and see how you go - I’d be interested to know. Then results might be surprising.
As best I can ascertain from the photo, the inside bottom surface has a channel around the edges. If this is not the case and the internal base is entirely flat, then there is another and even better potential solution. Simply cut off the feet of each container. A hacksaw or small cutting wheel would do the trick, with a bit of sand papering to give a neat finish.
