OK...first off, depending on your location, acquiring a sheet of 6mm x 60" x 120" Sintra is the first step. Have the Height cut down to 50", leaving the width of 120" as is. That will provide you with a 130" 2.35:1 Format Screen. With the w6000's H&V Lens Shift and Zoom, Constant Image Height is available, and the only masking you'll ever have to do is at the side when switching into 16:9
These could be removable Black covered panels, or simply Black Velvet Curtains on Travis Rods on each side, ready to pull in to mask the inside edges of the 16:9 image.
Mounting it on the wall is next, and mounting the Sintra directly onto the wall is best, using a Elastomeric Adhesive Caulk...about 6 tubes worth, spread out smoothly after being applied in 1/4" beads top to bottom, spaced at 1/2"
The Sintra should lay flat...and now comes the painting.
For a dedicated room like yours with such an ideal paint scheme, a excellent White could easily suffice. But if ever...ever you deign to turn on any lights, contrast will suffer.
One in your position need not worry too awful lot about having a screen surface specifically intended to combat ambient light (...and certainly not reflected light...) however painting a well done "Light Silver Gray" surface with 1.3 gain could only give you all the more leeway under any reasonable ambient light situations. You would not have to be so "intense" of someone request a little illumination. and they will...count on it.
Such a screen can & will still deliver accurate colors and terrific Whites. better still, the gain available will allow you to run that BenQ Beamer on low lamp, and in "best" mode...and either a decent Avia, Video Essentials, or Disney WoW calibration to the light shade of Gray, things will be optimally as good as they can be under a variety of situations.
Ahhhh...but what "Silver Gray"? RS-MaxxMudd LL
That's a mix you can either Roll on or spray (...spraying is the BomB...and much preferred...) Spraying is inexpensive too!http://www.avsforum.com/avs-vb/showthread.php?t=1398335
So now the Sintra is painted. Unless you paint your rear wall the same Black as the ceiling and simply "Float" your Screen, you'll take 1/2" x 3.25" MDF Base and Miter Cut 4 sticks, then wrap the Trim pieces in Black Velvet. (...same stuff you'd make the Curtains out of...$7.50 yard...)
The Trim then simply butts up against all 4 sides of the Sintra, and is attached to the wall using 1.5" Bright Finish nails....and hitting studs is not important...you drive the nails in at opposing angles to "cleat' the trim against the wall. You "set" the finish nails so they are just under the Black Velvet. Here's a tutorial.http://www.avsforum.com/avs-vb/showthread.php?t=865218
Consider this route as far as the masking requirements, The Curtains could be motorized & automated, but even with that, the savings on the Screen alone will allow you to consider the best choice of hardware, yet the job of designing a masking system will be immeasurably easier since it would only involve Side Curtains.
I like the potential of your Theater...and I've wanted to try focusing on some applications that would utilize the w6000. It's advent of having H&V Lens Shift AND
being a DLP just might start me leaning toward using DLPs with a degree of frequency not seen since 2005