# [APL (Dyalog)], 43 bytes

<!-- language-all: lang-apl -->

Requires `⎕IO←0` which is default on many systems.

    '#',{6⍴(≢⍵)⊃'0'⍵ ⍵(2/⍵)(∊⍵,¨⊃⌽⍵)(⍵,⌽⍵)⍵}1↓⍞

[Try it online!]

`1↓⍞` drop the first character (the hash)

`{` apply the following anonymous function

 `(≢⍵)⊃` use the length of the argument to pick one of the following seven values:  
  `'0'` a zero  
  `⍵` the argument  
  `⍵` the argument  
  `2/⍵` two (`2`) of each (`/`) of the argument (`⍵`)  
  `∊⍵,¨⊃⌽⍵` the flattened (`∊`) argument (`⍵`) followed each (`,¨`) by the first (`⊃`) of the reversed (`⌽`) argument (`⍵`)  
  `⍵,⌽⍵` the argument (`⍵`) prepended (`,`) to the reversed (`⌽`) argument (`⍵`)  
  `⍵` the argument

 `6⍴` repeat elements from that until a length of six is achieved

`}` end of anonymous function

`'#',` prepend a hash to that

[APL (Dyalog)]: https://www.dyalog.com/
[Try it online!]: https://tio.run/##SyzI0U2pTMzJT///qG@qp/@jtgkGXI862tP@V6srq@uYPerdovGoc9Gj3q2aj7qa1Q3UgSwFINYw0geJaTzq6ALSOodWAGUf9eyFiAEFoGwgrjV81Db5Ue@8/0BT/yuAQRqXMhecZYBgmiCYaQhmoiGSYiRxS0sE2zDRAsExMjJGMigNSYuhkTGSdSnmZuYIXlJSkimSpYlJSclI9oIAgmtsbmaSisZFtcbE1AzJDRYgCAA "APL (Dyalog Unicode) – Try It Online"