This was done in Fusion 360. We originally planned for a 6mm wall, but later had to thicken them to about 8mm. All of the walls have an outward draft. This proved especially important when making the original mold - had they been more upright it would have been very difficult to get the wood box out of the plaster poured mold.
This box forms the inside shape of the plaster mold to be made. The finger joint is the most popular technique in laser cut joining at 90-degree angles. But it also works with near 90-degree joins (although sanding is required to smooth the overlaps). The key to making finger joints is creating interlocking tabs with a cut depth equal to the thickness of the material stock being joined. Fusion 360 has features for woodworking that simplify the design and joining geometry. This box is made from 13 pieces that fit flawlessly together, it was amazing. Glue held it all together very firmly. After sanding we varnished the whole surface and glued the box down to a varnished board and added retaining boards to hold the plaster-containing form for the next step.
Make a mold for 4-gallon stackable calciners