Check out the AudioCircle Forum for GR Research. GRR owner Danny Richie specializes in OB speaker design, and has written much on the subject. Also check out the website of Linkwitz Labs. Famed x/o designer Siegfried Linkwitz offered (RIP, but I believe his products are still available) kits for a couple of different, very well regarded OB/Dipole loudspeakers.
Erik is absolutely correct, the distance between the front and back of each driver determines the frequency at which dipole cancellation begins. As far as OB speaker construction, Danny Richie has criticized the Linkwitz speakers for having baffles too resonant; the baffle of the midrange and tweeter drivers is a single layer of unbraced MDF, as is the OB/Dipole sub W-frame. The plans for the W- and H-frames Danny provides for his OB/Dipole Sub (in conjunction with Rythmik Audio) specifies double layers of MDF (or Baltic Birch plywood) for less resonance in the frame.
