I have always used this type of split conduit and clips for wiring in the engine bay . They are really neat and as you remember stuff and add things just open the clip slide in another wire and pop it back in,
http://www.thetoolboxshop.com/0-326-...nw75-965-p.asp
When you need a cable to exit the conduit just put a small hole in it on the other side of the split. when 100% done just tape in shut every 300 mm .
Bob