Just an idea but could you not invert the sink mold!
Caulk the bottom of the form spray or partially pour, then float the inverted sink mold into it. you will need to brace the back of the sink mold as you fill the remainder of the mold so it wont float up and move about.
I hope Im putting it across so you can follow.
Can you secure the sink mold from the bottom side using screws?
I would pour it using an ECC mix. I used silicone where the joins are on the melimine, let dry then screw the mold together. Once the it is removed then hand polish in the rounded edges.