Rabbits are a bear to do as it is. Myself, I would tell him I don't work on novelties ... but that's just me. I've done novelty mounts in the past, but not anymore.
It will be tough finding a mannikin in a position even close to sitting up like that. I don't recall if there are any up on the hind legs.
If you opt to carve the body, the "arms" will have to be
really reinforced, as will the torso!
One or two threaded rods running up into the body of the mannikin and fixed in place with a good epoxy, can be secured to the wooden base at the Rabbits bottom so there is no tipping or spinning -- as one threaded rod might allow.
That's about all I've got to offer in the way of advice. All I can say here is: "Good luck on your mission, should you choose to accept it." ... from the TV series
"Mission: Impossible" 
John.
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