I think Im right in saying that Outhill provide avons seating. I know they do backrests on their own - have a look on their website for a price.
Failing that, try ringing Fairweather marine. If you send them a cardboard templete they should be able to make what you need. Think their standard price is £132+vat.
I think the problem you'll find is that you won't be able to open your jockey seat as it hinges back, so it would have to be a removeable one to allow you access to the jockey, or possibly a side hatch could be fitted?