I've been thinking about dust collection for my new shop. For those of you watching, you may recall that I am planning to put in a wood raised floor over the concrete slab. I would like to run dust collection under this raised floor. If I used 2x6s on edge, I should be able to get a 5" pipe under it which would be sufficient for my shop. I'd like to explore the possibility of using 2x4s on edge to gain a few inches and cut down on cost a bit. If I do that, I obviously would not be able to use a 5" pipe. My question is, would it be proper/efficient to have rectangular ducts (maybe with rounded corners, or, even an oval shape) that carry the same amount of volume as a 5", but are shorter? I realize I may need to have these fabricated. The cost involved in doing that may kill the idea but I was wondering if you guys think that would work. Am I being clear here with my question?