The arrow side is big than another side, in this way, light tunnel can collect more light and focus on the DMD. It increase the brightness of projector. Amazing! only valuable projectors use this design and it cost more. When you intall it, The big side must face to lamp. 

I repair projector also, so I kown what material is the best. If you need other parts, please feel free to ask, I would be happy if you can send message to me.