In these photos you can see the progression of going from the stock wrist to an axial rotating wrist assembly acting as a plain bearing. Pardon the Orgrimmar welding - it's a cheapo welder.

The process involved cutting the bolt head off then grinding smooth the threads and then sliding on a stack of washers and welding the last couple washers into a mushroom head end stop then welding the other washers to eachother and these ones are to spin freely. They will do the pronation and supination. This replaces the need for a ulna and radius for that purpose, simplifying the skeleton some.

The metal outcroppings I left on the washers were meant to jut out significantly to give the fiberglass something to bite onto well for a dependable attachment.

Note: The stock skeleton does rotate axially already at a spot just near the elbow but that rotation is stiff and requires significant force to get it to move and loosening it is something I don't know how to do. I don't even know how it works at all. Advice on that for future reference would be helpful.

Note: There is too much clearance on the stack of washers so they can slide distally or proximally a good 8mm which is not okay - too much play. I need to fill that gap and lube it all with white lithium grease.

Note: I'm planning to probably just go fiberglass wraps over and over onto the stack of washers to grip it tightly and build outward from it and then go out and around the welded mushroom cap and then wrap onto the ABS wrist ulna/radius fused section.

Also note that I have reconsidered adding a dual hinge to the elbow joint and lean now toward just leaving it stock. I think the double hinge would add complication to the bicep attachment and cause some issues I'd rather avoid. A single hinge is easier to deal with IMO.