I've tried sneaking while putting down the mold, choosing it, and then trying it again but didn't work. When I finish the mold I have tried both right clicking, holding right, and pressing the left button and punching it until it breaks. Even tried sneaking and breaking it. As for following the voxel grid, I didn't see any specific shape I had to make it (I'm just assuming it's free-form) so I shaped it multiple times (in the screenshots it was just random).
I feel like I'm missing something very, very small. I was going to attach a video but that's past 39mb so I just uploaded what I did in screenshots.