Just a word of warning with the lids - they don't always make a good seal - 28 L sounds smallish so your snake may be able to push its way out of the lid.
There are some options - depending on how safe or sorry you want to be: You can do nothing, and check from time to time that there's no easy way out. Or you can use bulldog clips (I do).
Make sure you drill plenty of air holes all around the container.
I use heat cord on most of my containers - they're 50L and bigger - so only 1/4 of the tank is heated, the remainder is not. But I live in Cairns so it very rarely gets too cold.
I got a 15W heat cord, it does the job.
I know others who use heat mats directly. But make sure you use a thermostat then.
Good luck
