Hello, I have recently acquired an SI from my father-in-law for my birthday,(Great Gift).
I am going to be building a monaural system for my living room. I have to put the speaker close to the wall, so I've been working on this.http://www.hawthorneaudio.com/photos/displayimage.php?album=lastup&cat=10061&pos=0
24" x 17" x 37"
I know it doesn't show it in the picture, but the sides of the cabinet are completely open. The back of the baffle would have dampening material to stop reflections, and the defector would be highly reflective and filled with sand. I'm hoping to build a mule when it gets a little warmer.
I thought about useing similar cabinets to your pictures for my B200's, but I don't know how they get away with no damping material behind the baffle. My biggest concern with useing those cabs with the SI is the 15" driver moves a lot more air than an 8" and I think it would really muddle the sound. I'm no even sure my idea will work.
I remember you Birthday order from you Dad-in-law... Diana and I though that was cool.
I like your idea and can only offer the following as this is a bit out of my area of experience..
Make that back wedge a nice curve. I you make "stops" at both side... you could flex a piece of thin 1/8" plywood into these and thus create a nice curve. Once glued it should be fairly strong.
You could also make half round braces that run horizontal. If you preplan this well you could use the driver cutouts, cut in half to form these 1/2 round braces... Back filling the entire thing with sand will be very heavy and maybe not needed? I would cover this in felt and maybe drop a sand bag or two into the cavity.
You are in essence then creating a poly cylindrical diffuser and should see good results.
We have a couple of examples in the builder gallery of similar projects.
Both builders reported very high satisfaction with their projects...
Good luck with yours.
PS You have a cool Dad-in-law.