HI John

The SM204 is very much what you have already as a backrest / backband and not sure whill offer much more support over what you have but maybe a little. 

We do have a Flex 4 Seat that comes in the current Brittany 16.5 which has a much higher adjustable seat back and can be adjusted forward or back as well.  The price is $350 (says sold out but we do have some more coming a few days so check back) 

and we do have installation instructions are here: 

Hope this helps,