Using a HRV/ERV Fan without mounting it on a wall/window?

DIYers,

I’m considering various options for home improvement and improving the home economy and resource/climate efficiency. One area of focus is the fact that I live in Denmark and it can get quite cold, and so HRV and ERV fans seem like appealing options, especially the single-room ones, as adding an entire ventilation system is expensive.

The point is to ventilate the area while retaining the heat generated from indoor heating in winter, as airing out using the windows (the only other ventilation source) runs the issue of heat loss and inefficient air exhaust if I’m not generating a draft from another room, causing heat loss.

Before that, however, I am a tenant, even if the rules are a bit different where I live from the usual private situation (effectively a form of state-regulated, yet non-government public housing organization called “Almen boligforening”).

I’m not very experienced in DIY work, but I’m also not the only one I know nearby capable of contributing to a project like this in terms of carpentry/electric engineering capacity. I would like to ask here first for reference, though.

I am highly uncertain whether or not I would be allowed to make any large-scale home improvements, especially those that can be seen on the outside as a permanent modification to the aesthetic, as the local organization really values the “rustic” aesthetic.

It is also worth noting, as I update this post, to note that this is not the first action I would want to take for home improvement, noting things like weatherization and consumption optimization, but those seem to be a lot easier to have answered through Google than what I’m asking, as anything I ask just comes up with more generic “How to install a HRV fan” videos.

And so, with that as context for an answer, I ask the following:

Is it, from your views, practically feasible/efficient, in terms of cost-savings as a result, among others, of the less-airtight seal of a dismountable wall? for me to make some sort of heavily insulated wall that can be dismounted from a window in a jiffy, with one of the single-room heat/energy-recovery fans in them, plugged to an outlet, easily removable if I receive complaints?

I’m not sure I’d be allowed to glue it on, and so I’m not sure if there’s much of an easily-removable adhestive option that would give me a tight seal to maximize efficiency, yet also allowing me to dismount the construct following complaints.

If there’s something like this already offered in the market, I’d probably want to know as well. It would likely take DIY work, but would reduce it, and would probably be a more efficient product than what I would expect to make by hand.

Thanks for the answers in advance.