What are you looking to seal the foam chest with? Something spray on like Leak Seal or Plastidip, or something brush on like Mod Podge or Flexbond?
As for the seams - if you have a rotary tool, you can lightly go over the seams to try and make sure both edges are level and flat, but if not, don't stress - you can still patch them up with some Kwik Seal, or by using foam clay and sanding it back.