This may sound stupid, but...

[Silverzippo93]

I've been thinking. And I've come to a conclusion.

Expanding foam can be used for a mold!

If your Great Stuff was flush with the piece you're molding, it would dry to make a one piece mold....maybe....

But my question is, what would I use to seal it?

 

[colmon 9]

wouldnt it expand until it would crack the mold?

 

[silvercookie]

great stuff messes everything up cause it expands then shrinks and messes up the paper. i think frizzlefry tried this and it fed it up bad. there is another foam thing but i forgot what it is called. sorry.

 

[xalener]

I think that was because he used the wrong king of "great stuff".

 

[Digital Demise]

What If you put it in a trash bag while it is setting up but not very liquid.
You could then attach a shop vac or other suction device to make sure all the air was out.

You would probably need a filter on the vac though like a piece of cloth.