I modded a front mech into a chain guide for my DH bike back in the day. I cut the outside piece off the mech firstly. Then got 2 rectangle pieces of perspexs (spelling). Got 2 chain ring bolts (the outside part, not the bolt part), drilled 2 holes in the front mech, both on the inside shifting part thats left, one at the top/front and one at the rear/bottom. Drilled the perspex and bolted the perspex to the mech through the holes putting the chainring bolts in between the perspexs to allow the chain to run through it.
Hope this makes sense?
I have a photo of my bike with it on that i'll try and find.