But given that it's so small, what would the gravitational forces be if you made a shell with a 100km radius? Or pick a number - surely at some point it might be practical to build something.
Size of the object doesn’t matter. Gravitational force depends on its mass and how far away you are from the black hole. Standing on a shell ~6300 km above a black hole with the mass of the Earth is equivalent to standing on the Earth itself.
I have a hard time really accepting this concept given how unintuitive it is to me but I found your comment helpful in that regard.