I got creative with this part. Initially, I was going to make the cushion from scratch, buying foam/sponge and sewing together a fabric casing for it before sewing it onto the frame.
After a very brief search (I mean very brief, I didn’t travel far nor did I check out arts and crafts stores), I came across two potential shortcuts. First item I found was a ready made thin seat cushion (the ones that you place on the bum area and you sit on), and the second item was a puppies bed. Both were quite cheap and both were ready made cushions with fabric covering it!! This was going to save me time and money. In the end I opted for the puppies cushion and made a few adjustments to it before sewing it securely to the frame. It works a treat at 4dollars!