Call a major museum in your area, or a state history commission. They should be able to point you to "best practices" websites or perhaps offer advice directly.
Just heading out to clean / protect without some research can make for a good-looking box with no collector value because it was cleaned. True for any older item. Sometimes the dirt goes with the story.
Your correct, so far I vacuumed the dirt out of the inside and a light wipe with a damp cloth on the outside.