Complex mesh to 2 part mold

Hi there. Been using Rhino for mac passively for around 2 years. I am attempting to make a 2 part gun mold to be cast out of plaster. I know Rhino isn’t great for meshes and when trying to transform to a polysurface my computer crashes. I have been playing around with fusion 360 and have been able to successfully cut the shape from my mold but am getting error messages when trying to do a uniform fill for 3D printing. Attached you can see my issue, the program is reading all of the bullets/ other inner mechanisms of the STL. My question is how should I go about simplifying my asset before moving to fusion 360? I was looking on the forum and played around with the draping and some other tools as I just need the outer superficial surface and none of the inner stuff. Any help would be greatly appreciated! Thanks

Now that needs some explanation!

Hi Ben - no file is attached - probably too large - please zip and upload to Rhino - Upload to Support, with a link back here in your comments.

-Pascal

https://www.rhino3d.com/private/download/3a2d4777-9809-48f9-af14-5e5cd44eadf1

Hi Ben - what is the goal here, to merge all the parts into one and then split in half?

-Pascal

Yes!

any pointers?