There might be a better way, but here's what I know:
One way it to fill your MeshInstance with custom primitives, like boxes and spheres, until it is covered sufficiently.
Another way is to select your MeshInstance. A "Mesh" option will appear on the top toolbar (next to Transform and View). Click that and select "Create Convex/Trimesh Collision Sibling".
Note that I have tried that and for some reason it slows down my game to the point of crashing. Others say they did not face any issues. Could you please try it and report your performance?