UPDATE: This tool has moved to: https://www.shift-e.net/tools/#!/matte-generator
After reading Scott Simmons' article on inserting visual timecode (aka BITC) onto your sequence, I posted a comment about the FCP part of the article suggesting an alternate way to put TC on your sequence without having to nest it. I don't really like nesting, so my method just involves putting the Timecode Generator plugin on the matte I use to display my sequence in its proper 1.85 or 2.40 aspect ratio.
I thought it'd be fun, if maybe only marginally useful, to make an online dynamic matte generator. This little tool creates a PNG file at the specified size and with a black matte to letterbox your content to the specified aspect ratio. Slap it onto the top layer of your FCP sequence, and if necessary, drag a Timecode Generator on top of it.
And one last thing, the option below for a 16:9 matte just produces a completely transparent image at the specified size. With 30 seconds in Photoshop you could produce the same thing, but this is even less work than that.