A custom model that makes Falco look like the best to ever do it.
(Easy) Open an Akaneia iso in mexTool then go to fighters, Falco, Costumes, then add the costume .zip file with the add from file option, then re-export your iso.
(Harder) Open a Melee ISO in mexTool, click yes when asked to install mex. Click file -> export as -> file system and select an empty folder somewhere on your computer. Now in mexTool click file -> open file system and select the folder you exported to, then click select folder. Go to fighters, Falco, Costumes, then add the costume .zip file with the add from file option. Now export your ISO and optionally delete the file system you exported earlier.