I use a Hue bulb with an E27 socket. The bulb socket can be screwed to the base part (holes 60mm apart). The side parts are inserted into the base part and fixed by the lid. The lid is a combination of a side part and the file for the lid. These are aligned in the middle and glued together. A milky acrylic glass plate (20x20cm) is glued to each inside.

In addition, I printed small bars to prevent the light from shining through at the edges.

It was printed on a Geeetech A20M with a base plate of 25x25cm

A smaller version (~16x16cm) can be found here: https://www.thingiverse.com/thing:4259034