This is a remix of the sand castable mold box sides from OlFoundryMan on YouTube;

https://youtu.be/cX2u6S5qV3Q?si=fFpg-y-_EvvSaP5S

Watch his video series on the topic, you will learn a lot.

I recreated them from his PDF drawings, though not exactly to scale or dimension.

Breaking the sides up into smaller pieces should make them easier to cast, less likely to fail and those with smaller printers can print any of the pieces.

I've included a casting runner that incorporates the small wedge to help when you are packing the sand around the sides.

Some post machining will be needed. Drilling holes and threading a few of them for screws. For a full two part mold box you will need eight center pieces and sixteen end pieces.

Once you've finished drilling and threading the holes you can configure any size box you want. A center piece and two sides is needed for each side of the box.

If I were better at using Fusion360 I would fillet the edges, but that is not the case. More than happy if someone can do that, I will repost the files.