Hi Nick, I've attached a 3DM version of your model that you can open with MoI.

But it has the object as a bunch of separate surfaces, it's going to be fairly difficult to get it all joined together into a solid due to several little slivery surfaces in there.

Little slivery fragments make things much more difficult to join because it's kind of easy for the opposite side of the slivery bit to get fused on to an adjacent piece. One thing that can help with this is to join little slivery pieces to their smallest sized neighbor surface first before joining that to the larger pieces.

What is it that you intended to do to this model after importing it into MoI?

- Michael