Celtic box, before printing, in the slicer, increase the model at your discretion to the size you need. Print 4 pieces of each part, then connect the walls with patterns into the wall mounts, inserting them into the grooves, or glue them. I'm not sure much, and I think that when printing, the wall fastening grooves can shrink and become smaller, so I advise you to slightly increase them in the slicer program, by a couple of percent, from the actual size you selected for printing the Celtic box.