I blocked the window in my darkroom with heavy, black plastic that you can purchase at probably any hardware store. I will likely be with the paint drop cloths or in the garden section as it is frequently used in gardens to prevent weeds from growing.
This is probably going to be more light tight than most any fabric that you can buy.
But the plastic can drop off or droop without prior notice.
The OP is looking for a solution which will not have that problem. That is the same reason that I cut plywood added black-out cloth and handles to my window cover.
Warning!! Handling a Hasselblad can be harmful to your financial well being!
Nothing beats a great piece of glass!
I leave the digital work for the urologists and proctologists.
I don't see (and haven't found) any reason why black plastic would drop or droop without prior notice. It's much lighter than fabric and easier to secure with tape.
Cut it to the size of the window frame, and tape it around all the edges with strong masking tape or the like. Never once had a problem; and if your windows are the right kind, you can even retain the ability to open them up while maintaining light-tightness when they're down!
When I had my darkroom in Salt Lake City , I had it set up in the spare bedroom, had a nice setup for the window, it was a slider rail from a cabinet door set in the window frame, with 1/2 inch particle board cut to fit, 2 pieces so I could slide one over and have daylight if wanted , had weather stripping at the edges to block any light from the cracks. This worked out very well for many years.
One more item that I didn't see come up. I am not too sure about just how warm the weather gets in Hong Kong, but you may need to invest in a air circulation vent or fan. If your new darkroom is small, real estate in HK is not cheap, you will need some kind of air circulation. There are powered vents available that are designed for darkroom use. It is important that the air circulation system have some kind of filtration system. HEPA filters or carbon filters should do the trick.