Autodesk Inventor

[#1]

Hey guys, I am working on a project and need some help figuring out a way to fill a concave space I have with a seperate solid that takes up the exact amount of space. I would like for this new solid to be its own part. I will post a picture so you can understand what I am reffering to. As you can see, there is a concave space in the middle of the design; what I am looking to do is fill this area with a type of foam, so I need to model something that fills all the spaces up to the top plane. 

I have tried using the thicken/offset tool but it gives me errors on vertexs and such and I have tried many different selection variants. I did get the space to fill in but then ran into the problem of it being all one solid, not something I could take out and make a different part, which is what I am looking to do.

Hopefully this makes sesnse, if not, feel free to ask any questions. I will post my ipt file for those who need it.

Thanks!

[#2 · Accepted solution]

Is something like this what you are after?

[#3]

Yes! That will work perfectly, thank you very much for your help.