John,
I just completed the mod with the barrel under the dust collector. I used a fiber barrel (free from work) with a metal snap-lock lid. I built Phil's seperator and just bolted the metal lid to it as I was doing the final assembly. Due to the forming on the outside of the metal lid it is able to hold the weight of the seperator and with the locking band it seals tight. For stability I added a piece of 1/4" hardboard to the base.
To remove the barrel, I just loosen the 6"x4" rubber connector (about $5 in the plumbing depatment at the non-orange box store) and slide the PVC pipe into the barrel. I used part of a fitting mounted in the lid to create a good seal and still allow the PVC connector to slide up and down. The blue mark on the PVC tells me it is in position based on Phil's design.
This has worked great. I have no way to measure this, but I have not noticed much loss in performance and it is much easier to empty. Last week I used it a bit too long and filled the barrel to the bottom of the baffle, but it worked right until the end. This is why there is dust in the bag in the picture above. The next mod is to add a window. While doing this I added some of the quick connect fittings to the hose and made life much easier. Hope this helps.