Sort of hate to say it, but it looks like that box could have been made with sawdust mixed with glue. Neat if so.
As far as secret compartments, look up details of some of the old roll top desks. Some of those had over a dozen different secret compartments. That should give you some ideas. Puzzle boxes sometimes have secret compartments also.
Dustin Penner has made some jewelry boxes with secret compartments and made YouTube videos of the construction. You might like one of his methods. He hasn't posted anything lately, but made quite a few interesting woodworking videos in the past.
here is a link to my FB page showing the tops final assembly - amboynia veneer spiral match
on my educational website https://www.imaginegrove.com/ I will be posted the complete build soon.
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