The top has to have mortised ends, to fit in the groove in the sides. We cut the mortises first.
<a href="http://www.flickr.com/photos/happymonkey/95981881/" title="Photo Sharing"><img src="http://static.flickr.com/23/95981881_fbc11cd8dd.jpg" width="500" height="375" alt="Starting the Top" /></a>
The top has to extend past the front of the groove, so the mortise has to end before the front of the board.
<a href="http://www.flickr.com/photos/happymonkey/95982924/" title="Photo Sharing"><img src="http://static.flickr.com/12/95982924_8540bac994.jpg" width="500" height="375" alt="Top Ends" /></a>
We used the tablesaw to cut down the router groove, and tested it in the assembled frame. It was a bit tight, so we shaved it down a bit with an edging plane and a chisel.
<a href="http://www.flickr.com/photos/happymonkey/95983932/" title="Photo Sharing"><img src="http://static.flickr.com/32/95983932_0c2bff3611.jpg" width="500" height="375" alt="Fine Tuning the Fit" /></a>
We fixed up the mortise with a small saw cut and a chisel.
<a href="http://www.flickr.com/photos/happymonkey/95984839/" title="Photo Sharing"><img src="http://static.flickr.com/27/95984839_0c26240dc7.jpg" width="375" height="500" alt="Trimming the Ends" /></a>
Sliding the top into the frame.
<a href="http://www.flickr.com/photos/happymonkey/95985977/" title="Photo Sharing"><img src="http://static.flickr.com/38/95985977_d22f9e74e3.jpg" width="375" height="500" alt="Sliding in the Top" /></a>
And here it is with the top on.
<a href="http://www.flickr.com/photos/happymonkey/95987017/" title="Photo Sharing"><img src="http://static.flickr.com/34/95987017_1044c1dbc0.jpg" width="375" height="500" alt="So Far So Good" /></a>