How can I conceal windows without any adhesive or replacing the vertical blinds?

Install blackout curtains. Hotels use them a lot for the comfort of their guests who may want to sleep during the day. You can order them online. And, curtain rods are not that hard to install. Curtain rods come with their own hardware. You will just need a screwdriver.

You may consider electrotropic foils or electrochromic glass. Both share that their transparency is altered by applying a low voltage current.

• Using an electrotropic foil requires that this special foil is attached, similar to scotch / adhesive foil to already existing plane glass surfaces. An elder demonstration (prices reflecting the UK back in 2017) is shown here, a larger application / home improvement here.

• You may opt for electrochromic glasses like SageGlass by St. Gobain. Here, the material responsive to the electric current resides within the glass (actually, multiple layers [architecture]). Because it is glass, some variation of colour may be added, too. Of course they made commercial-like presentations like this one, but are not the only on the market (e.g., example, example). In this case, you may get rid of blinds altogether. An other brand name is SwitchGlass.

Have you heard of static cling?

Some kinds of plastic film will stick to glass without adhesive using only static electricity. When you do not want the effect, you peel it off and roll it up for storage. Unroll it for reuse. Easy on, easy off.

You would be most interested in Light Block Window Film. There are all different kinds of window films—UV blocking films, IR blocking films, and various kinds of patterns and opacities up to total block out. You might even be interested in gradation from clear to opaque.

Solid white can also block light from the outside and has the added benefit of a bright interior with normal room illumination if you don't want to open blocked windows.

The stuff can be found in major hardware stores and home renovation centres.

Good luck

My hack is to buy or make a light-reducing box around the bed instead of trying to mask the windows.

Following the idea of this privacy indoor bed tent, which might not exclude enough light, you could make your own light-proof box around the bed.

Build a timber framework held together with bolts and wing nuts. Staple same thick fabric to it, and make a ceiling too. Don't forget to allow for some ventilation.

Six months later prise out the staples and dismantle the frame, fold the fabric and store away for next time.

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Assuming you have difficulties sleeping when the sun shines even at night, consider using a sleep mask instead of blocking all of those windows. I often sleep with a mask in summer and a well-fitted one stays in place all night long, so you can put it on in the evening and still be protected in the morning.

Unfortunately there are vast differences in quality between different masks.

• Make sure it fits you very well around the nose and cheeks. That's where you'll be most bothered by light.
• Make sure the material is thick enough to block light. Black fabric is the safest bet, but even a very thin black mask will let some light through.
• Make sure it's comfortable enough to sleep with. Some masks are quite bulky and might hurt you when sleeping on your side.
• Make sure the strap(s) is/are adjustable to your individual size. If you can handle a needle and thread, you can shorten the strap with a few stitches.

I ended up sewing my own sleep mask, which is a very easy beginners project and lets me adjust the mask to my preferences perfectly.

Vertical blinds have 2 problems:

1. they are not entirely opaque,
2. there's a gap between the window and the blinds, which means quite a lot of light gets in.

You can get cassette blackout blinds (random example) which solve both problems. The cassette is fitted in the window frame to ensure there's no gap for light to get through, and the material is opaque.

Because they're installed in the window frame, they won't interfere with the existing blinds.

But it's difficult to find a flexible material that's opaque enough to block out the sun completely - I've got blackout blinds and I can still tell when the sun shines directly on the window. The amount of light getting in is very small - the difference between pitch black and being able to tell the room has a window. The ultimate solution is metal roller shutters on the outside of the window.

e.g. 6 months

Well, 6 months is not 3 days. What you need is not a life hack, but a proper solution. Replace all the fancy blinds with proper blinds.

Alternatives:

• use the fancy blinds during the dark 6 months, and the proper blinds during the light 6 months;
• have both blinds installed; use them as you see fit. E.g.: if you have a night party during the light nights, you can use the fancy blinds.

For this, you need top contact a business that deals with the stuff - they have the mechanisms, as well the opaque material for you.