Can someone write a step-by-step guide to making a SOLID helmet?

I own several helmet models and armor kits from here, including the commercial license to sell them. I'd like to shrink some of the helmets to be one inch tall and make them solid inside, but I'm having a hard time finding a good process to adding 'bottoms' to the helmets so they remain solid.

I've tried MeshMixer, Fusion360, SketchUp, all to varying degrees, but ultimately pulling them into a slicer finds some reason that they're not solid on the bottom.

I'd love to hear any ideas from others on how to accomplish this.

cheers,

ian