Removing Mesh holes

Sorry if the title is an incorrect description for what i am trying to do.

But i am trying to remove these holes in the model, however this is a very complex model and has waaaay to many polygons. I think this is because it was modeled in solid works. I did not generate this model but i would like to modify it. Any tips would be greatly appreciated!!

I would think reducing the polygons would be step 1 but the mesh is triangulated. I did not have much luck with using alt+j on this, i am newer to blender and do not know all of the tools yet.

Here are the pictures:
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Select the vertices that for the sides of the holes or just select everyting that encompasses the holes. Then fill the space by selecting the vertices that form the boundary of the hole and Alt+F to fill. Yo this for the top and bottom sides of the object.

Not a bad idea but that is messy and leaves internal faces, verticies and edges. It also does not match the mesh density of the remaining mesh.

If its flat, make it 2d (delete all but top vertices), select boundary loop, (F)ill holes, extrude back.

It is not a flat object, this is a case for a Raspberry Pi 2.