It seems that this question was asked in the forum a while ago. But I'm not sure if you're wanting exactly the same thing. I can't figure out how to get the link to the thread itself, but this is BB's solution. You can look at the rest of the thread to see if it's a solution to your particular issue, I guess. :)

http://www.renderosity.com/mod/forumpro/?thread_id=2887216#msg4185232

In order to maintain aspect ratio, I did this, but I used a big square with a rectangle of the correct aspect ratio cut out so that I'd get a frame across the top as well (rather than 2 squares as BB suggests for the person who is wanting just a darker frame on both sides). Works like a dream for me. 

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PoserPro 2014, PS CS5.5 Ext, Nikon D300. Win 8, i7-4770 @ 3.4 GHz, AMD Radeon 8570, 12 GB RAM.