I don't see the need for an immersion bag, but I don't agree that a simple plastic bag is appropriate either. The OP is, and rightfully so, IMO, concerned about possible ESD damage from sliding the laptop into and out of a plastic bag.
While it may be remote, there could be a problem with the plastic bag. Sliding objects into and out of plastic bags can generate many 1000s of volts of static electricity. Just touching the laptop's exterior case [probably] would not be a problem, but you would not want that voltage discharged through a contact in one of the laptop's communication ports (USB, Ethernet, graphics, etc.).
If you slide your laptop out of a plastic garbage bag, and don't have Earth ground, a metal filing cabinet or a dog's wet nose nearby to discharge any static through, there is a possibility ESD damage to a laptop component might occur. So why risk it?
It is important to note that a Grand Canyon size trench (microscopically speaking) can easily be torched through millions and millions of transistor gates in high-density ESD sensitive devices from a discharge so tiny, we (as humans) cannot even see, feel or hear that a discharge (ark or spark) has occurred. So I would not take any chances.
Measure your laptop. Then buy
anti-static bags large enough to completely contain it. I would get bags that are resealable, then make sure the bag is inserted in your backpack opening end down. This will not only protect the laptop from the rain, but also static build up too.
The good news is, if you are in a location where you are worried about rain, it is extremely difficult for static electricity to build up in high humidity locations. So it is more likely getting wet is a bigger risk than getting zapped by ESD.
I would not leave your laptop inside a sealed bag (any water-tight bag) for extended periods of time, unless you are sure it is totally dry inside. Should moisture get trapped inside the bag, I would worry that mold/mildew might form. That would not be good. I don't think it necessary to remove the laptop from the bag (unless dripping wet), just leave the bag unsealed when not raining to keep it aired out.