Several years ago I met a dayan (judge) of the B'Datz (religious court) in
Jerusalem who wrote a book called Tiferes Chaim decrying the use of the
'square knot' as the double-daled knot mentioned in Kabbalah works. He was
able to research and determine the formation of a 'true' double-daled knot
as mentioned in the Mishna Brurah. His son(-in-law ?) showed me how to
create a double-daled that looks like two daleds, front and back, up and
down, with a hole in the middle as a closed (final) Mem looks. If anyone is
passing by Philadelphia PA, I could show it to you.
Anyway, the purpose of his book is to show that despite the popularity of the
four-square knot, it has very little likelihood to be considered halachicly
correct. At the end he has a copy of the decree of the B'Datz of Jerusalem
recommending sole use of the single daled, because everyone agrees that it is
halachicly acceptable to fulfill the mitzvah of Tefilin.
[NOTE: please consult your religious authority for the final word in any
Halachic matter, especially if your family custom has been to use the
somewhat common double daled. -- YM]