SQL Server - create a function that takes an int as an argument and returns a formatted hexidecimal based string.

I need a Transact SQL function that works as follows:

func_x(1) returns '000000000001'
func_x(255) returns '0000000000FF''

In other words, mac IDs without hyphens or colons.
Going from int to hex was easy, hung up on the rest.
Current SQL.

Thank you.