With one of these.
First I cut holes slightly too big in the foamcore, then I cut holes slightly too small in a sheet of flexible foam, then stuck them together. I think I made the latter a bit too small, getting the ships in and out is a bit of a pain, but right now you could pick that section up and turn it completely upside down without any of the ships falling out.
Tip: If you get one of those compass cutters, make sure you practice first. It took me a few goes before I could reliably stop the point in the centre of the circle from slipping.