the spoiler would have to come off to fit one, as they have to clip into the top of the boot panel, also you cant fit a towbar to it so you cant use the ones that sit of a towbar

i would say put it inside, Steve is your best man to ask!
_________________________
Red Escort 1.6LX
Then - Red Escort 1.4
Then - Silver Fiesta ST 170BHP
Then - White Fiesta ST MR200+
Now - Mitsubishi Evo IV Forged 2.3 500BHP
and Mondeo Mk3 2.0TDCI