allowing for different string lengths in message box

The following code gives a warning message but the different string lengths of the registration numbers is offsetting longer values

Sub a1()
Dim LR As Long, i As Long, msg As String
With Sheets("TRUCK SERVICE")

    Dim rCell As Range
    Dim rRng As Range
    Set rRng = Sheet1.Range("B3:b56")

    'For Each rCell In rRng.Cells
        'rCell.NumberFormat = "@"
        'rCell.Value = myFormat(rCell.Value)
    'Next rCell
    LR = .Range("l" & Rows.Count).End(xlUp).Row
    For i = 2 To LR
        If IsDate(.Range("l" & i)) Then
             If .Range("l" & i).Value - Date < 30 Then
                 msg = msg & vbTab & .Range("B" & i).Value & vbTab & "CVRT Expiry" & vbTab & Format(.Range("l" & i).Value, "dd/mm/yyyy") & vbCrLf
             End If
        End If
    Next i
End With
If msg <> "" Then MsgBox prompt:=Left(msg, Len(msg) - 1), Title:="CVRT Expiry Warning", Buttons:=vbExclamation
End Sub
Public Function myFormat(str As String) As String
    myFormat = Format(str, "!" & String(9, "@"))
End Function

I've tried quite a few methods, including the above function, but cant find the solution. Anyone any ideas?? Appreciated as its annoying