My account and posts from my previous tomato cube mold designs from Printables got taken down (probably VPN and/or Linux usage). Nevertheless, I found a better platform on Thangs.com. The search is much better and it searches other platforms as well as its own website.
I have linked my new and improved uploads with even tighter tolerances below for you guys to download. V.3 introduces a completely cube interior with none of that angled walls v.2 had.
V.2: https://thangs.com/designer/cad noob/3d-model/Cube Tomato Mold v.2-924366
V.3: https://thangs.com/designer/cad noob/3d-model/Cube Tomato Mold v.3-924368
The most optimal solution is rubber bands on the area in the middle section. This way, if the tomato decided to grow more than expected, the rubber bands will keep them in place or expand with them.