This has been complicated for me, because my crankset leaves very little space between the crank and the outer large ring. One portion of the setup is the commonly available Chain Disk:

http://www.koolstop.com/Accessories/

From top, the guard comes in two pieces designed to work with this kind of disk protector:

http://www3.big.or.jp/~number-1/No.1_Chainguard2.html
http://www.ogk.co.jp/amenity/MTBgoods/MG-001-10.html

Front part No. is MG-10 and rear MG-001, made by OGK, ~$10 each piece in Japan, a souvenir for me from a trip. I would be now eager to get more of those. Finally, the gap between the pieces is closed by a velcro strap, to prevent the leg getting caught in the gap, while leaving an access to the derailleur in case it were needed.