Pretty much this. The best way to be consistent and do as you are doing, handle it periodically for short periods. I think 30 minutes might be overall though. 10 minutes a day or every other day would likely be sufficient to garner results, but it will take time. You want the experience to minimize stress, if you handle too long, you run the risk to doing more damage than good in some cases. This is pretty general though as you mention simply that you got a snake "who doesn't seem to have been handled much" and "I have 2 females and never had this issue with them". This is very vague and does tell us much about what problems you are specifically having.
Does the snake bite? Is it overly squirmy?
In birds or mammals, a common suggestion is to place the animal or animal's enclosure in a room with heavy foot traffic which allows them to have constant exposure to human activity, but they can view it from a safe location. I have not tested this theory in snakes or other reptiles, but I can verify that they do learn via positive reinforcement and generally tame down over time with human interaction.