Need help printing a simple empty-box-like object

[Clupus]

Hi folks!

First time posting here, so please excuse any mistakes.

I'm trying to print a "question-mark cube" from Super Mario Bros. I've already printed several, no problem.

This time, I want it to be hollow on the inside with a separate lid, like a small box.I've modeled it in Blender and it looks perfect there (image attached). But when I slice it in Cura, it becomes "closed", with a lid on top (image attached). I can't get rid of that, for the life of me. Already tried setting infill to zero, but nothing changes.

Any thoughts?

Thanks,