Parametric sealed gift box.

Use the Onshape file link and modify the variables to change the box size. Provided model is 34mm cubed inner volume with 3mm thick walls for a 40mm cubed outer volume. This is a balance of strength and usability.

I've also tried 40mm with 5mm walls (total outside size 50mm) at 20% infill using PLA which required a several hammer blows to open.

Add a pause gcode command just before the top wall starts printing to insert your gift – ideally a rolled up bit of paper with a QR code or similar.

https://cad.onshape.com/documents/35c3b25b0a51d890274920ca/w/fd74765fa08b7c7dc564b689/e/63086fe531ce9949e04a9190?renderMode=0&uiState=658dd7f772334c18f210b7bd