Just big enough for air-flow + lamp inside .

If somewhere where noise is NOT a problem

https://www.1-hydroponics.co.uk/fans-and-filters/extraction-fans/ram-inline-exhaust-fan-100mm-4-150m%C2%B3-hr.html

If somewhere where silence is important;

https://www.1-hydroponics.co.uk/fans-and-filters/extraction-fans/rvk-inline-extraction-fans.html

£40.00 mini-grow-tent/enclosure (make or build!)
£15.00 - 65.00 extractor fan
£17.60 10m LIGHT-PROOF ducting
£12.00 lamp stand for UV bulb
£ 8.00 UV bulb at 245nm

GBP £ 82 - 132; DONE.

https://www.1-hydroponics.co.uk/fans-and-filters/ducting/combi-duct-lightproof-ducting.html

Biggest problem; UV light escaping out of the duct = solved already by cannabis growers :) lols

Larger surfaces/air spaces;

https://www.screwfix.com/p/bwt-rl30-g13-t8-germicidal-uv-tube-30w-894mm-3ft/9293r

And the link for us, would seem to be;

https://media.ies.org/docs/standards/IES-CR-2-20-V1-6d.pdf

" Wavelengths in the photobiological
ultraviolet spectral band known as the “UV-C,” from 200 to 280 nanometers (nm),"

£ 8.00 UV bulb at 245nm; perfect thus.

i.e. we want short-wave; .a.k.a.: UV-C

2.4 How does GUV work to disinfect air?
Commonly used GUV lamps generate predominantly 254-nm UV radiant energy, which is close to the
peak germicidal wavelengths of 265 to 270 nm – both in the UV-C range,

What TI's would seem to need = section 3

3.0 Disinfecting Room Air with GUV

:)

Notably, even if not possible to install GUV;

"3.3 Is GUV inside air ducts sufficient for room-air disinfection?
When UV is used in ducts, although it ensures that recirculated air does not have viable pathogens"

If just sterile air is required, the above hack would seem to work to provide it. :)

doesn't stop transmission .. unlike GUV but ... if you live alone and/or don't have high enough ceilings ....