[Amps] power cord strain relief recommendations?

[2 at dock.net (2)]

I drill two 3mm holes about 30mm from the hole where the mains cord comes 
through, and secure  the cord flat against the metal with a nylon cable 
tie.
>
>I am rewiring a couple of SB-220 amps for 220 and need to replace the
>existing cords with 12-3. Anyone have any recommendations for
>type and sources of appropriate strain reliefs?
>
>cheers, Paul