The egg crate itself will limit the movement of most of the Chaeto, you may still get little pieces that move through, but snails and hermits will often make quick work of that in your DT (Just be observant - you may have to remove bigger pieces that begin to grow in places they don't get to - like powerheads). Additioanally, you can place a sponge between the two large chambers - that complete eliminates any movement of Chaeto, but sitting the bottom of the sump like that it will have to be taken out and cleaned on a very regular basis. Otherwise it will collect debris and begin polluting your tank with nutrients. As far as the HOB, if you have the refugium/sump set-up, I personally would ditch it (but I also hate them, so its a personal thing), however keep in mind it does provide you with additional water volume and may provide a place for heaters and filters, so you will have to weigh the pros and cons.