My speakers combine Tympani 1-D bass/mid panels with locally made 6' ribbons, which screw in between the bass/mid panels. I find the imaging sharper with the tweeter closer to and in the middle of the bass/mid panels. The imaging is as sharp as cone speakers, but with all the other advantages of panels.
My key design parameters were:
To address these parameters, I chose to build the frames from a fine grain Australian hardwood, Jarrah. This is a very heavy wood. I chose 50mm (2") dressed planks for the frames, 200mm (8") wide for the sides and 100mm (4") wide for the "open" legs and top and bottom frame edges. The solid "wing" support leg was made from 50mm (2") Jarrah. These wings are 1.2m (3.5') high and about 400mm (16") wide at the bottom. These wings are very heavy!
One of the features of these frames is the way the panels are held in. No screws! I routed a 25mm (1") groove around the inside of the frame. This groove is 25mm (1") deep on the sides and bottom, and 50mm (2") deep at the top. To insert the panels, I just slip the top into the top groove of the frame, position the bottom over the bottom groove, then lower the panel into the bottom groove (like fitting a sliding door into a cupboard). I then slide the panel sideways until it is inside the groove in the side of the frame. The panel is now sitting with 25mm (1") of its frame covered by the groove on one side and top and bottom. When I do this with the second panel, I am left with a gap of 75mm (3") in the middle of the panels. The ribbon tweeter fits into that and screws into the back of the bass/mid panels. To make it all secure, I force 6mm (1/4") rubber tube, normally used to secure flywire in aluminium screen doors, into the gap between the panels and the frame at the back. This locks the panels in tight.
The frames have all been joined using biscuits. Great invention! The frames for the screens were made from 3mm x 30mm (1/8" x 1.25") stell flat, welded at the corners, with the cloth stretched and glued over them. Timber was finished in a one pack polyurethane satin flooring finish.
The hard work was in shaping the edges of the frames. To stop them looking too chunky, I wanted to shape a curve on the sides of the frames, a bit like the edge of an aircraft wing. I rough planed these with a hand plane and then did the final shaping by hand using a 300mm (1') length of sandpaper stapled to a flat block. I hand to sand "with the curve" in the same way you sand a curved fender on a car to avoid getting "flats" on the curve.
This time I used some left over Meranti from an F18 hardwood lintel. This was dressed, sanded, edged (with a router) and finished with Danish Oil. I then screwed some 300mm (1') lengths of pine to the top which had been ripped down their length so that they just slipped inside a 100mm (4") PVC tube. The tubes were cut to 180mm (6') and covered with black auto carpet, glued on with contact cement. Careful trimming of the carpet allowed a seamless finish on the carpet.
The carpeted tubes were slipped over the uprights, with a little contact cement applied to the insides. To ensure they didn't move, The tops of the tubes were drilled 150mm (6") from the top and threaded length put through the holes with chrome tubing used as sleeves. Locknuts and washers inside the tubes allowed me to lock it up tight. The close-up picture of the tubes was taken before the chrome spacers were polished.
The only problem is that when the lenses are in place behind the panels, you can't even tell they're there. All my work making them look good was for nothing!