This is the design for my HT surrounds. They are designed to be placed in the back corners of the room. They fire upward. Based on experience with a test box, these boxes will be just as effective as wall mounted speakers and will hid well in the corners, in and amongst the furniture.
The drivers are RS 40-1354a's. The boxes are 20 liters, ported QB3, tuned to 48 hz. F3 (of the test box) is 53 hz. The height is the same as the height of the arms of my sofa (!). The two braces are positioned by 1/3^0.5. I tried the golden ratio, but didn't like the size of the largest unbraced area.
The boxes are made of 3/4" mdf. Originally I had intended to round the edges to 3/4", But ultimately, I left the edges square. The sides are finished with oak veneer. The baffle does have 3/4" round-overs, and is finished with black truck bed liner. The grill is a sandwich of 1/4" mdf and 1/4" of felt.
Here is the finished product: