[James Romeyn]

System will employ two 8" tall x 4" wide planar dipoles in a vertical array, 260 Hz - 6 kHz.

Preference is to build a U-frame for the planars; rear radiation to be damped by a high-density foam pad to be adjusted in-out from the planar's rear surface depending on front wall spacing. 

What are recommended dimensions for the U-frame housing the planars?  In case height matters, minimum is estimated 20". 

[ttan98]

Use Martin King or John K software, visit their website.

Their s/w are quite cheap.

[el`Ol]

12" should be a sufficient baffle width, a u-frame won`t be necessary, unless you want to reduce the dipole character.
http://melhuish.org/audio/baffle.html