In the end I kitbashed Harald Deathwolf and a techmarine together as they are both awesome models and I had them lying around in my closet of doom collecting dust.
For the rider I began by sawing the techmarines legs off at the waist and doing the same to Harald. The torso form the techmarine and legs from Harald were then pinned together and the joints filled and sculpted back in with green stuff. Then I chose a couple of arms for the servo harness and put the rest aside as I wanted to keep a few spare for my next iron priest conversion (yes I have started another already, I am on a roll). These were then positioned to help balance the techmarine model a bit on the wolf, and finally tubing was either drilled into place or sculpted on to add more depth. Came out pretty well I think :). What are your thoughts on him?