MAC at @reboot, like some cellphone

I want to change my MAC at reboot, so making it a cron job like the following in BSD.
Can I do this in the jail for the user, setting it as a command or should it be a script?

I would set it as a command

openssl rand -hex 6 | sed 's/\(..)/\1:/g; s/.$//'

just to test it, it works.

To make things easier, blinded by the light, I just took the macchanger script from my linux distro and set it already unpacked in the folder of

/home/name of the user/sbin/

setting the flags and doing it at or @reboot.

It works fine with just the command line!!!!

@reboot  usernamesomebody    openssl rand -hex 6 | sed 's/\(..)/\1:/g; s/.$//'

I am asking this, because the BSD handbook tells me to try it first as a homebrew script in the really
special environment, that is giving me a headache like the following

env -i SHELL=/bin/sh PATH=/etc:/bin:/sbin:/usr/bin:/usr/sbin HOME=

And my second question is, would it be better to set it in the system crontab

/etc/crontab

or just user crontab?

So if someone can help me out, thanks in advance!!!