Rajesh,
He is not suggesting to keep L/R at 21 ft apart. Just spread them enough to make the perfect 60 degree or so. With a listening depth of 10 ft, you may have to spread these around 12~13 ft. This configuration is suggested to get a wider sound stage compared to what you would get with speakers spread out max 10 ft wide.
If you do want to put the screen on 10 ft wide wall, then to get a wider sound stage, you would have to spread those max possible, that means at corners. Baffle wall is recommended to get rid of the room boundary interference.
One point to note - I checked out the speakers you planning to build. They have the port on the back. Since you will have these speakers close to the wall (irrespective of baffle wall or not), move the port to the front. Having the port in the back may cause issues.
He is not suggesting to keep L/R at 21 ft apart. Just spread them enough to make the perfect 60 degree or so. With a listening depth of 10 ft, you may have to spread these around 12~13 ft. This configuration is suggested to get a wider sound stage compared to what you would get with speakers spread out max 10 ft wide.
If you do want to put the screen on 10 ft wide wall, then to get a wider sound stage, you would have to spread those max possible, that means at corners. Baffle wall is recommended to get rid of the room boundary interference.
One point to note - I checked out the speakers you planning to build. They have the port on the back. Since you will have these speakers close to the wall (irrespective of baffle wall or not), move the port to the front. Having the port in the back may cause issues.