久しぶりにVBA書いたよ

Option Explicit

' 文字長を指定して、乱数で適当な文字列を作成します
Function get_unique_name(length As Integer) As String
  Dim offset As Integer
  Dim base As Integer
  base = Asc("a")

  Dim result As String
  result = ""

  ' 指定された文字列分だけ連結
  Dim i As Integer
  Dim r As Double
  Dim c As String
  For i = 0 To length - 1
    r = Rnd()
    offset = Round(r * 100, 0) Mod 26
    c = String(1, base + offset)
    result = result & c
  Next i

  get_unique_name = result

End Function