I
think a 650 will fit a nook-sized cover (with an inch or so "extra" at each end), if you velcro the back to keep it from slipping (glue'ing the actual velcro pieces to wide tape, to make them removable, if necessary);
it should remain within the corner straps so as to look decent as a work-around. Here's the link to the available designs (that red dragon is snorting "Buy me already!")
http://www.oberondesign.com/nook.php