Woodsmith has some plans for a Morris chair and ottoman if that is where you end up.  In the magazine article at least, there is a section on how to add the cushions. There is an online extra from the magazine article with a video about adding the cushions too.    This might be applicable to whatever style you make. 

If you lived in my area (Plano, TX), I have a good upholsterer who has made some excellent cushions for us and the price was fairly reasonable IMO.  You have to buy your own fabric which is where the cost comes.  If you check with work rooms used by interior designers, sometimes they sell remnant rolls that are enough to do one or two chairs.  

--Nathan, TX. Hire the lazy man. He may not do as much work but that's because he will find a better way.