Quick update on this after 2-3 weeks
The box has bowed quite severly and cracked at multiple points
Maybe the effect would have been minored by cutting along the wood’s fiber direction, but overall, I think cutting out the whole center inside the base and lid has a significant effect on material stress, and this method is not sustainable , unless you have very thick walls 
Oof. That is a shame. Stupid wood.
Wow, that sucks…
Wood does weird things. My first reloading block curled up like a banana when the holes were bored with the rings curving up. When the holes are bored with rings curving down, they stay flat…
Sad, but a nice bit of learning for us! Thanks for the update.
I think it’s ‘just’ differential movement - the dark timber has dried and shrunk across the grain- the light has nowhere to go. I really would not have expected that on something so small. What is happening with your house heating?
I think that fully encapsulating it in polyurethane might have prevented this.(might) . For utilitarian stuff like this these days I use satin wipe-on poly, final buff with steel wool or scotchbrite then wax to make it less plasticky.
I wouldn’t have given it a thought except that on the boat when we were based in Lorraine I fitted some Ikea laminated benchtops and came back after one winter to discover they’d grown 19mm! 
Messy!
