If I can offer a couple of thoughts... I couldn't find the right thickness of aluminium once for a Linhof board I was making, so used a slightly thinner piece and ran a line of black electrical tape around the edge, mitred at the corners. It provided the right thickness and was light tight. Maybe easier than routing a thicker piece. Also for this board I superglued a piece of 3mm mdf on the back for a light trap. All spray painted mat black, worked well. I'm sure you'll do a great job of it.