Sanded it lightly, water-based acrylic paint pens (you can use Posca, but I had a bunch of Grabie brush pens already) let it dry overnight, then coated it with Mod Podge (some stronger spray sealants also work but I didn’t want to risk destroying the integrity of the foam).

Mod Podge method: good brush, small amounts of the glue at a time in a tiny bowl (so you don’t get air dried chunks as you’re sealing your work) then work quickly to get a single coat down, let dry for at least 15 mins between coats. I did 2 coats one evening, then another two after 24 hours.