I need a 3D modeller

I need help from someone who's expert in working with 3D meshes. I'm trying to merge two models into a single one that should represent a lava tube. I'm doing this for numerical modelling and i need to obtain a final solide stl model to import in comsol that shall represent a volume with an inner cavity (the lava tube) and several holes at its surface. I actually have a 3D mesh representing both the surface of the area of interest and 3D mesh of the inside of the lava tube.