Have a look round the back of your internet router and see if it has a USB socket. If so the simplest option may be to plug a drive straight into it. Depending on the storage requirements you might be able to use a USB flash drive. 

Thanks but I’m looking for a simple NAS for storing documents too but want to be able to use it for Sonos too 

What does the router offer, in terms of a USB socket for shared storage?

It has a usb but as I say I’m after a NAS 

A USB stick plugged into a router that has a socket for it is the simplest kind of Network Attached Storage that can be used for Sonos. And for documents.

^^^ This.

I used various standalone NASes for some time, but they proved a bit troublesome so I moved over to a 256GB flash drive inserted into the router. It was sufficient for my local online needs, as I prefer offline USB drives for backup.

Ok thanks I’ll see if I can get that to work, but how do I find the files from the router?

Test it out with a spare flash stick. What router is it? There should be instructions somewhere online for how to access the shared folders.