Hi Derek,

> I tried to put the feature in the box and then used
> "Boolean - Diff" to make a mold. It didn't work.

The boolean for that case should work, but it will produce your shape as an internal cavity inside the block - that does not produce the mold for you yet but you are partway there.

Draw in an additional cutting line in the Top view and then boolean the piece with the cavity into 2 halves to make your mold pieces. Or you can also just include the dividing line as a cutting object in the first boolean as well, that should work also.

- Michael