Help with filling hallow map

[KrimsonWoIf]

hello. i made this map in my game using a low poly method with brushtool and building tools but the inside of my map is hallow. i currently have a spawning script that spawns coins and it keeps getting trapped in the hallow area. i know filling the hallow area will fix the issue but i dont really know how. i cant fill each area in manually becuase the map is huge and there is almost no single place that is flat. i have tried using gap fill but it goes crazy and 4 out of 5 times it clips through my map.

any suggestions on what i should do? Im mainly looking for a plugin or something to make this a lot faster than it has been.

HERES MAP:

INSIDE OF MAP (hallow area that needs filling)

thank you if your able to help.

[Cogenize (Cog)]

That looks like a 3d model, To fix it, import it into blender (or whatever 3D model software you used), select the bottom sides/verticies and select all the top sides/ verticies and press Alt+F to fill the hole, hope this could help