Tefilin shel rosh knot

According to Rav Abadi, what kind of knot is preferable for tefilin shel rosh for an ashkenazi without specific custom ?
simple dalet or square ?
the square one seems to be the old minhag (yekkes use it till today) but the dalet is widespread

When my son became bar mitzvah around 20 years ago I also purchased teffilin for myself as well from the Rav.

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