The mid drivers I've seen in this design are always off set toward the inside and a little closer to the top than the bottom. My mid baffles measure 19"H X 18"W and the center of the Tang Band is 11" from the left side and 10" up from the bottom. I don't think these measurements are all that critical. I used black painted molding to cover the ugly plywood edges. With stained high grade baltic birch ply these could be quite handsome. Had I known how good they would sound I would have gone that route.

I would definitely try the Dayton 8 inch Point Source ( PS220 ). It's SPL is 95.6 at 1W/1M which is a little higher than the TBs but it's Qts is a little lower.