Possible Rondo Substitute?

Hey everyone, I have a quick question. I've never used the rondo method for helmets, but I've come to the point where I need to use it for a prop as I cannot access the inside of it for fiberglass matting. There will however be an opening at the bottom big enough to allow me to dump something inside to help harden the piece. I don't have any Rondo, but I recently came across some 321 Smooth Cast I forgot I had. I have both part A and B of equal amounts. My question is, can I use Smooth Cast as a substitute for the rondo? I'll still be able to resin the outside of the prop to seal up any holes of course.

Thanks.