I popped into my local Travis Perkins and bought a sheet of clear perspex for about £5 and then cut it down to fit...
Held it up to the side of the case, marked where I needed to mask (as you can see it took me a couple of goes

), then just sprayed it with normal Black Car spray from a can...
I'd bought some sticky velcro strips from my local Lydl's ages ago so just used that to fix the perspex to the side of the case, at least doing it this way I can pop it back to normal anytime I want...
Andy...