This has been on the back burner over the past few weeks - as has most of my modelling, frankly - but I started work on the base today.
I bought an oval of wood at the local craft store and varnished it last week. This morning, I mixed up a batch of scenic groundwork powder, along with a little white glue (to prevent cracking), and some red, yellow and tan acrylic paints for colouring. Now, you'll notice the curving coastine on the boxart, and this was roughly drawn onto the wood. The plaster mix was spread over the base up to the coastline, and shaped as per the boxart - there's a small cliff/wadi area in the right foreground that is covered by the kit name on the boxart, but, trust me, it's there on the original painting.
The sea was painted with a medium blue, and is a fairly close match to the boxart, and this was covered with a coat of Future. A little playing around later and here's a rough mock-up:
And with the model:
The backdrop will be the top half of the boxart - I'll print this a little larger and longer on thin cardstock, rather than this paper mock-up. The background aircraft will not appear on the backdrop - I'll use 2D cutouts that will stand out from the backdrop.