I too find the Omega Optical/DGM NPB filter to be the one essential for deep-sky observing (link below).
One handy accessory is a filter slide. My homemade one is shown in this photo. This removes the possibility you'll drop your precious filter in the dark. This slide also has an O-III filter fitted, but the NPB is the one I use 80% of the time.
http://www.omegafiltersebuyer.com/se...ers/Categories
Phil