So I'm back looking at this again, and I wont be able to cover the holes with a plate so I am going to have to fill it.
I am taking out my old CD player as it looks rubbish with all the new Lowrance kit I have installed this summer, and I have managed to get a cheap Sonic Hub with blue tooth so wont need anything mounting on the dash anymore
From all the information everyone has given me on here I am happy filling the hole, but unsure about the final white finish - What is the best product, and how do I get a match?