Additional notes:

* Files added to the /custom/ folder should be stored using the same path as they would if you were to add them to the /tf/ folder.

Screenshot example:
* .tff files will not work in .vpk
FletcherD commenting on tff incompatibility with vpk[github.com]

Noting this either:
Do not pack your HUD files (if they have custom font)
Or do not pack the .tff files
Or pack the hud and live with default fonts

* Packing your files into .vpk is recommended but not necessary

Mounting a VPK to the filesystem is more efficient that adding a subfolder,
because each time the engine neds to open a file, it will need to make a call to the
operating system call to search the folder. VPKs can be searched by the engine much
more efficiently. Each subfolder is a new search path that must be checked each
time the engine tries to open a file. So for optimal load times, always use VPK files
and don't make any subfolders in this folder!