Holy cow! You're a lifesaver!

Thanks so much for looking into this. Yes, I do have Windows 11 and downloaded Microsoft 3DBuilder. I used it to fix my own model (with the lid) and it fixed the STL perfectly to be able to slice & print.

Here's the final outcome:

Cube.jpg

Again, I have no idea why Blender is breaking these models when exporting the STL, but I'll look into the 3D toolkit feature you mentioned.

Thanks, once again!

Cheers,