Well I am not going to give you a design to go with but I will tell you to keep in mind that there is only a 3 inch clearance between the ring and the spider foot. I will also give you the thought that angles work very nicely for circular objects such as rings. lol those are my helpful clues keep them in mind and I hope they help
Edit: This is of course talking about your arm designing part of things, the rest of your design looks pretty solid just make sure you have agood weight distrubition.