So Ive made an assembly, and now I want to house it in a box by hollowing it out. (Box will be 3d printed)- basically making a mold.

Im trying to do this with the indent feature as I want a 0.5mm gap all around so it's not too tight a fit.

Ive made a solid cube and added it to the assembly and then I clicked 'edit component', on it, then features, 'indent'.

It just wont let me select the assembly to cut in the tool body region.

Ive also tried it by making the assembly a part but it wont let me select the entire model on the tree display, in the tool body region.

Ive watched several YouTube videos trying to work it out but they all really basic things like a single chess piece.

Anyone had experience with the indent feature to make a mold?