I do like this and use a cord from the wrist to the shoulder in order to stand the weight of the wole arm. I put a plastic piece glued to the foam and a knot in each end of the cord. A bit of hot glue on the knot avoids the knot to untie.

This is what I do (or rather what The Foam Book suggests). I add a piece of the doll's joint inside the shoulder to attach the rope to - which threads through the foam arm - and then the other end attaches to the hand. It's the only time I use hot glue: gluing the knot at the 'hand' end means that with some needle-nosed pliers and careful removal, I can swap the hands, change the puppet's costume, do repairs, or keep the hands/rods separate for easier storage.

I do agree though that the hand joint does end up being seen slightly, so I usually either hide it well or will cover it with some extra skin fabric.

I have used some bouncy balls inbetween the thigh and calf of smaller puppet legs for bare leg puppets to give a more rounded looking knee, but it adds a little weight so I don't do it on puppets that will usually have clothes on. I took a large thick needle and pushed it thru the ball with heavy thread that also threaded thru the rest of the legs to hip and ankle where it was secured. I did not try it on the elbow joints, I think keeping the puppet as light as possible was more of a concern. I was trying to simulate a real knee cap look.

Snail, you could do the same with a foam ball which would be lighter. You could even take a piece of sheet foam and cut into a circle ant then round of the edges.  Should give you a nice knee cap look.