You are approaching your prototype build and your casting design is not complete. How many of you have been in this situation?
One course of action is to quickly finish your design and hope for the best. Another possible solution is to contact your prototype supplier regarding your needs and share with them your design issues. Often the prototype source may be able work with the partial design (staying away from areas of potential change) and save time for you. The prototype source may also be able to design the tool in a generic form and allow secondary machining to complete the part when the design is firm. Unfortunately, not all prototype processes allow this, however it is worth investigating.