How to fix a ventilation pipe to the wall?

[Vilx-]

I'll repost my question from DIY StackExchange in hopes of getting more answers.
I'm doing some renovations in my toilet/bathroom, and am currently pondering the ventilation. Here's the current situation (click pictures to get bigger versions):

My toilet and bathroom are right next to each other (yes, they're tiny. It's a Soviet-era building). The bathroom is on the left, the toilet on the right:



The toilet has a 10cm by 20cm hole in the wall between it and the bathroom:



On the other side of the wall there's a metal pipe placed in the hole:



The pipe spans the width of the bathroom and enters a similar hole on the opposite wall:



That's where the ventilation shaft is. You can also see a smaller hole beneath the pipe - that's the ventilation hole for the bathroom itself.

What I want to do is place fans in both ventilation holes (for the toilet and for the bathroom). The only fans I could find where of this type:



The smallest one had a diameter of 10cm; it's depth was greater than the thickness of the wall between toilet and bathroom (the rear end sticks out a good deal on the other side); and the face-plate (being a square) doesn't fully cover the hole in the wall. I even had to knock the holes in the wall a few mm bigger for the fan to fit in there at all; and the current metal pipe is about 1cm less wide than the hole, which means that the fan cannot possibly fit in it.

That pretty much means I'll need a new pipe. I don't mind, because the old one is pretty unsightly anyway. The new pipe will most likely be circular (I haven't seen rectangular ones like this in the store) and unlike the old one it won't be able to support itself by being placed inside the hole (since the hole is barely wide enough for the fan, there's no room for the pipe). That means that it will need to be somehow attached to the wall and the remainders of all the holes closed.

What is the best way to do this? I can imagine buying the right length of the pipe (which will then need to be very precise in length), and maybe three metal (or plastic?) plates with appropriate holes cut out... but how to stick it all together? And it would also very much then depend on the pipe being the proper length (exactly the width of the bathroom). A few mm longer or shorter, and it's unusable. That worries me.

Is there some other approach or some tricks that can help me?

[Greg]

Here in the US the pipe (duct) would be galvanized sheet metal, that forms a tube.  They come in various diameters like 3", 4", 6" etc.

I would use two pieces, one long one that slips into the other short one near the end to prevent sagging in the center.  The joint would be secured using 3 small sheet metal screws and foil tape to seal the joints.  I would then cover the duct with a light frame and gypsum wall board.

[Spork]

It's even common in the US to run this fully inside drywall (with no sheet metal).  This seems to vary by area.   However, I don't think that's a good idea in a humid bathroom environment.  But you could, alternatively, run flex duct or even dryer venting and box that in with drywall.  These would hook in nicely with the fan's round vent.

[Vilx-]

A drywall, unfortunately, is not an option. These rooms are already tiny (the bathroom is 2 square meters; the toilet even less) and the precious centimeters that a drywall would take up would be sorely missed. :(

[Vilx-]

Actually, I have another question - are these bathroom fans available in other form factors as well? Upon closer inspection of the bathroom hole (aka the ventilation shaft) I found out that I can't put the standard fan in there at all - the hole is just too shallow.

A crazy idea - could I maybe use a standard PC case fan (plus appropriate power adapter)? They are pretty cheap, come in variety of sizes (including many small enough to be placed in the hole), and I could even fit two in the hole. I don't need timers, moisture/light sensors and other extras. The fan will anyway be simply connected to the lamp, because I don't have any other power source in there. But will the moisture of the bathroom not present any problems? Will the airflow volume be large enough?