Wood Sealer

I am building a wood display enclosure out of plywood (birch). What is a good wood sealer that is safe?

fiberglass resin. Use a solid scraper to smear it on. Its nontoxic when it dries.
And it will make it waterproof forever.

Something less involved is a good polyurethane. After it cures over 24 hours it is completely safe. It is even rated food safe for prep surfaces in kitchens.

I used a sanding sealer then minwax polycrylic sealer on my beardie's custom enclosure. Just make sure to let everything cure for a week or so. After that it should be good to go.