Diagonally rhymes with:
additionally, conditionally, congressionally, constitutionally, conventionally, criminally, educationally, exceptionally, fractionally, functionally, generationally, institutionally, intentionally, internationally, irrationally, marginally, medicinally, nationally, nominally, nutritionally, occasionally, operationally, originally, personally, phenomenally, professionally, proportionally, provisionally, rationally, regionally, rotationally, seasonally, terminally, traditionally, unconditionally, unconstitutionally, unintentionally
Diagonally sounds like:
decamillo, decennial, decimal, diagonal, dicamillo, dismal, dismally