This is a hard one Alex.
When I had my 4" f5 achro I couldn't find any eyepiece to do what you want and ended up using a 30mm plossl that gave 3º FOV from memory. It was the only one that had a fairly flat field. Tried ES 28mm 68º and was pretty bad half way out and was only 3.8º... RKE 28??? looks bigger than it is, also not 2" as you know.
