My plan is to design and build two limbs for poor Houdini.
The first will be a basic one for him to get back up and running... so to speak.
This will likely be a simple wheel design.
This however will have a major flaw which I will approach in the second version. Whilst this fixed appendage will allow Houdini to get around with level poise and without dragging himself along the ground, it would leave him with always being propped up at an angle should he ever wish to just lie flat.
Version two will likely still be a wheel, but will also be spring jointed. The spring will be designed to provide the perfect level of resistance so when Houdini lies down, his weight causes the wheel to push out and up, but as he steps up on his three remaining legs, the drop in pressure will cause the wheel to push down into position and hold it's corner of his body up, keeping him comfy and level.
I am going to try and use my housemate's DIY 3D Scanner to get a good starting point for his body shape and go from there, but this may just turn out to be insufficietnltly accurate.
Anywho, I'm giving Houdini a preliminary "screening" sometime over the next week and will take it from there