I was looking for the effective joint parts.
A good item for the plastic model kit is being sold by Japanese maker.

• -Wave http://www.hobby-wave.com/LINE_UP/optionsystem/index.html
• -Kotobukiya http://www.kotobukiya.co.jp/item/page/parts/index_poly.shtml
• -Yellow Submarine http://ys-hobby.shop-pro.jp/?mode=cate&cbid=182565&csid=7

These items has some types, size and length, shape.
There are various types of these items.
There keeps strongly.

However, the only problem is a high cost! XD

I used 6mm Double-Ball-Joint.
http://www.yellowsubmarine.co.jp/hobbybase/ppc/ppc_top_waza.htm#double

The 6x6x7mm cube was used for the hole.

I was using the form of a cylinder first.
Because the cube touch the ball a little, the adjustment is easy.
I recommend the cube.