When I worked in a fabric shop the owners mother used to make 'cute' sofa doorstops from a brick.....
they had a small teddy affixed unfortunately we had several returned because they went 'mouldy ' I think the brick started to sweat in a warm house .. and lots of peoples houses are very warm ...so we had to refund them.
'I would wonder what might happen to cat litter if it got damp, and some types do give off dust.'
that's why I double wrap... I have one in my living room its survived the last 10 years and has a zipped removal cover which gets washed