Negative and positive space within sculpture

I wanted to experiment with the positive and negative space of the new work, but not be able to see the negative space is difficult to visualise how this would work.

It’s hard to look at the weave and not see how it will look when the plaster is no longer there. Being able to visualise the finished piece is becoming more and more difficult.

If I can’t see the negative space or void in the sculpture how will I know that it creates an interesting juxtaposition with the positive or the woven surface of the sculpture?

Do I weave and hope that it works when cast? There must be a method of allowing me to experiment with this combination without waiting until the work has been cast.