What can I say? Women collect shoes, I err, collect sunglasses!
First I counted all my pairs and added 2 for when I inevitably buy more pairs!
Then I found the largest pair and the smallest pair and offered them up against each other.
So I marked a piece of 6mm MDF board out to the correct width, 14cm wide in this case.
Once I made sure the initial concept worked, I could then cut the board. Again, holes etc are always easier drilled on a larger piece of wood than a smaller one.
Some sort of spine bolted onto a base plate would be the best bet and I found the perfect solution. An old spinning drinks optics rack had a secure heavy metal base plate with 2 holes in the bottom! I wasn't going to use it, so it was donated.
To secure it onto the base plate, I used 2x 4" long wood screws and a dab of PVA glue for good measure.
A quick check and it stood up perfectly!