This is the smaller display I've been using for testing -

110_MatrixDisplay by mc_mtb, on Flickr

As you can see, when it's dark it's nice and clear, however during daylight you can't really see the lit LEDs.
Here's the display in daylight, with what we bodged over it to make it visible (it's not powered up, as I'm not sure where I've put the controller for it at this present moment - I tidied up last week!)

LED Matrix by mc_mtb, on Flickr

Vinyl may work, and I know where I can get plenty offcuts. Not sure why I never thought of that.

However I've discovered the plasticshop do samples, so I've ordered some of their frosted samples in dark shades (midnight black, slate and stone grey), as it should give me the finish I'd ultimately like to acheive.