I had to redo the crossbeams on the bottom to make the frame wider. This allowed room for the warp engineer to man the cannon on the top platform. I also added a couple pieces of styrene rod to act a spacers and support arms for the cannon.
Here the crew are added and the wheel just placed on. The engineer is manning the gun and the two skaven slaves are pushing the contraption from inside the frame.
I still need to add a couple more cross braces and a front plate with spikes. The base is a 50x100mm so it is right on the size needed. It comes apart so I will be able to paint it easier. The model itself is going to be slightly larger than the new kit but still close enough for me.