How to make the lid/top and not screw up mitres

[fleyh]

Just as an update - I have decided to risk it and resaw my box top into two 5mm thick panels. I am going to glue one of those panels without any extra allowance for wood expansion and see what happens in a year

The short sides of the box have the wood grain going in the same direction, so I am hoping they will expand similarly to the top and not ruin the mitres.

Thank you very much to everyone who took the time to read my question and make a suggestion.

 

[HamsterJam]

Both options sound very possible, but I am not sure I understand them 100% as I can't see how this will allow for wood movement because I would have to glue the top to the fillets/strips, wouldn't I?

Glueing strips to the inside of the box then fixing the lid to them won’t allow for movement.
However if you can cut slots, the strips can be glued only to the lid allowing the assembly to float in the sides. There may still be an issue of an expansion gap if the lid is fitted within the sides….

Excuse the poor drawing but yellow is lid and blue is fillets or strips glued to lid only and sitting in slots in sides. Red arrows point to potential expansion gap.

 

[fleyh]

However if you can cut slots, the strips can be glued only to the lid allowing the assembly to float in the sides.

You are a genius !!!
Thank you so much for the clarification and for taking the time to make the drawing.
This is the kind of solution I was hoping for.