In all my years on the forum I've never seen this done.
Of course it's possible but you'll need a good hand at bodywork to mould something into the existing cut out.
Frankly it's a waste of money and that £500 would be better spent elsewhere i.e.:
GFB DV+
New PCV
New Cam Follower
New headunit
Highline clocks
If you got really lucky you could get all of the above for the same money.