looking for advice on a silicone mold

I have a 3D printed part that I need to make foam replicas of. I used Smooth-On Rebound 25 and created a silicone mold of it. I then used a fiberglass like product (forget the name) that also paints on and solidifies.

I can't put the shell on as one piece because there's no way to remove it. Even a two piece is tough because of the inner part of the piece. I broke it up as a number of pieces but it doesn't provide enough rigidity and the foam I poured in came out a little warped, especially in the one place I didn't have the shell.

Any ideas for how I can put a shell on this? Pouring silicone into a box mold seems like a waste of silicone for this part. Can Rebound 25 even be poured?

Here is what I'd do:

-Cut an old silicone mold you no longer want in tiny fragments, mix them with thickened silicone. Use this to fill that nasty gap that prevents you from making a simple two part shell, until you get a more or less solid block (build a good wall where the opening is)

-If you don't have an old mold, try mixing the silicone with tiny fragments of something else, like cotton, bandages, foam.

This will save you some silicone