How to create a mold in a shape of a complex part, but only half of the mold?

Hi,

I need a "block" with the surface having the shape of a big, complex part, very similar to this one (but rotated 180°) https://www.callparts.de/sites/default/files/productimages/MAS11001215171033/2386656.jpg

The block, or mold, has to have the exact shape of the part so the part can be positioned on it very accurately.

As you can see, the part is very thin and has a complex shape, so subtracting the part from a block via formnest and then cut the block in half does not work. (Or does it? I'm quite new to SW).

My next idea was to extrude the part to one side and then to subtract it from a block. But extruding the part does not work, probably because of the weird shape and all the single elements ist consists of? (In addition to that, the part is importet from a CATIA file. Maybe that's the reason why it doesn't work)

Is there a way to get this done? I feel like I've tried everything, but nothing seems to work.

SolidworksPlastics And Mold Design