For the Box you could use XPS foam. Loading that many Barrels I think plausible to use paper cartridges instead of measuring the Powder manually for each barrel. Maybe a Box of small cannonballs and a Box of cartridges for the cannon and spyglass, loading stick and bucket for the crew?

You could make the cartridges by simply rolling up a bit of tissue paper and slightly diluted wood glue wood glue into a sausage shape. After drying you can cut of one end to create a sharper edge like in the picture.