Hi SweDerpZ
While Cartboard could prove to be sufficient to accomplish the task of hiding your PSU and it`s cables, it could prove to be a fire hazard having it inside your computer depending of the application and your case. Secondly paint become hard when dry, contrary to the cardboard that is flexible, if not handled correctly you could end up cracking your paint job.from an aesthetics standpoint, cardboard is not always even and can be easily be damage. I would not recommend it
Acrylic is a good choice as it aesthetically pleasing as it is but, will required additional tools and/cost to make it fit your application and if you want a specific color of acrylic can be challenging to get it in small sizes unless your local shops already have some spare sheets lying around (i tried to get orange acrylic but had to buy a huge sheet of a few foot x foot only when i only need a few inch x inch, and that had a cost of a few hundreds dollars),
their are cheaper kinds of plastic that can be used for this purpose, (ask a local plastic shop) like the one`s used for the hockey rings ( i forget the name of the plastic), i would recommend that specially if you are painting it as the point to pick acrylic is for it`s natural looks.
soft metal sheets could be a good options like aluminum they can be painted and are easier to work with (cut, bend etc..) to make then fit in your case.
Hope that help you,
have a nice day