This is one of better tips I got a long ago and now am passing to you. If you act on wha you will read here, you can save some money in the long run. The technique is called "blocking" and you can use it on all types of freezers, not just chest  freezers.

Sometime it happens that your freezer is not full enough and that there is free space available. This happens more often if you have a chest freezer that helps you store food during several months only.

For example, you have vegetables that you wish to store because the summer is over and there aren't any fresh ones. You don't have a cold storage room so you need to use a chest freezer to preserve those vegetables. But, if you can't fill it to the maximum, then you are losing money. It is simply not economical to use only 50% of your freezer. This is why you need a solution, and freezer blocking is exactly what you need.

The main idea is to fill up the unused space inside your chest freezer. Because there will be less air in the freezer, then less energy will be used to keep the temperature low. Whatever can keep the temperature low inside will help the compressor with the work and there will be less energy and work used in keeping the interior of the freezer cold. Air is not a mass and it can't keep the temperature low. This is why we need to put something inside the freezer, something that will preserve low temperature for longer period.

One way to do this is to put cardboard boxes inside your chest freezer and fill them with insulation. You can easily find some cheap insulation, like fiberglass. You can also use those insulating peanuts, but I think fiberglass works best. Then you have to close and seal the boxes all the way so they do not mix them with food.

Put those boxes in the empty spaces and fill that space as much as you can. More boxes, less work for your chest freezer.
There is an alternative method - you could also add containers of water to your freezer. This also works great, but remember to put the plastic bottles and never fill them 100%.

And there you have it. The colder the interior of your chest freezer, less work for your compressor. That saves you money by reducing electricity bill and prolongs the life of your appliance.

Now you know that the basic ides is to make your freezer works less. Put it in a clod room, without any direct sunlight, and this will also help with the efficiency. Close and open doors without hesitation so you don't lose the temperature. If you do all these little things, you will definitely notice that your chest freezer is working less then usual, which is a great thing and a big plus for you.