For the uninitiated: 3D printers use 3D CAD files for their source data (sort of). Laser cutters are strictly 2D, and AFAICT there aren't any awesome modeling programs designed for making 3D stuff on a 2D matchine.

Fortunately, OnShape (free online CAD -- highly recommend) has a tool to make a "laser joint." This is maybe what it sounds like -- for two intersecting parts (like, slabs of wood) this will cut "fingers" so they can be glued together later. It's quick-n-dirty joinery in two dimensions.

So here's how to make an OK box:

1. Make a variable for your material thickness. For cheapie hobby plywood, 3mm. 1/4" works well too, but is a little heavy for a small box.
2. model the thing to its final shape: for a simple tea box, make a rectangle the size of the outer dimensions (so "interior + 2x thickness"), hollow it out and all that. if you want a lid, make a lid. Same basic rules. These are solid 3D parts.
3. Model the slabs from the faces: this is DEAD SIMPLE: in OnShape, extrude the outer faces by thickness. If you're doing it right, these slabs will all intersect.
4. Use OnShape's "laser joint" tool on the parts which will be glued. It can make single joints, or has a pretty good "automatic" mode which can do an entire glued-up part. For a box lid this might be 5 slabs all jointed and glued up. Twist the knobs, there's some good stuff in there. For the example tea box there are two laserjoint features -- one for the base, one for the lid
5. For each of the "slabs" (10 total): select the outer face of the slab an "export DXF".
6. Import those 10 DXFs into your favorite laser software (LightBurn) and go nuts. Feel free to put some pirates or creative text on there.

This works shockingly well, considering how easy it is. Boxes don't have to be cubes -- LJ doesn't require it. Just be sure you extend the parts if you're making weird joints, to keep enough wood in the joint. You also don't have to make a box; check out my snappy switch stand!