I found this web site a while back when I was looking for designs to carve.
I was thinking more along the lines of "love spoons" than box tops. see:

http://www.wallace.net/knots/

It has instructions on how to create your own knots.
Also http://www.identifont.com/samples/ba...elticKnots.gif
and http://en.wikipedia.org/wiki/Celtic_knot
have some good images