Permanent is better but if it's just a few weeks per year you have to worry about, get a decent portable, and remove the water before storage.
Make sure it has a decent seal with the window. Also a lot of them are noisy - be aware that having it next to your bed might be an issue.
Best tactics:
DO NOT HEAT ANYTHING INSIDE.
Seriously, I see Brits complaining about the heat, then putting the kettle on. An oven is worse. Even a gaming computer is basically a small oven, laptops are better, but gaming generally pushes heat into the environment. Unless you play Solitaire, I suppose.
In a pinch, you can get ice delivered and if you can keep a container of ice in a bedroom, it should help. Sleep is the most important time for cooling, you can endure heat when awake, but you need sleep to cope, and you will sleep better with something like this.
https://ice-express.co.uk/order-online/
If you are going out, there are icepacks etc you can strap to your body. I just grab bottled water, let a bit out, freeze. I walk with a backpack and it does help to be able to splash cool water every now and then, and just having it against my back makes a difference. Freezer bag can be useful.
I've been posting solutions for about a decade on Tumblr and not once in that time have I seen anyone actually trying to solve their problem rather than complaining.
Yes, I know what it is like to be so poor you can't do anything, but most people are not in that boat. They can do something, but venting means they are no longer motivated to take action.
Just remember, an insulated house means it is easier to keep it cool.
Also, before I forget, large windows are bad, if you can cover them FROM THE OUTSIDE or at least shade them, that will help. I've seen some older homes with small windows, but sadly the global fashion in architecture is large windows which are a stupid stupid idea, but the psychology means they can make great profits.
