For creating the flats the 3 stones need to be the same size. I haven't ever seen anybody make a flat, but I did see my brother make a telescope mirror. He started with 2 flat round pieces of glass. The he put a drop of water and some grinding compound on the bottom glass and started walking around the stand he made for it. He did that for hours and hours, a couple of hours a day for many days. When he finished he had a spherical convex tool and spherical concave mirror. Both of them were extremely smooth, good enough that he silvered the mirror to use in his telescope. As far as moving a big stone, all you have to do is to get it moderately flat. You can do that with a hammer, which can be made of stone. Then you lubricate the surfaces between them. That way you don't need nearly as much force.

As far as the inside of the box is concerned. If you can make a small flat, or straight stick, you can use that to check the flatness of the inside of the box. Then you very carefully start polishing the inside frequently checking to make sure it is flat. Places where it isn't you concentrate on grinding flat. This is called craftsmanship. As I said, I have not seen anybody doing this, but I have sanded things smooth and flat. Actually I have sanded something curved, the rockers on an antique rocking chair. With care you can get extremely good results, just working by hand. There is no need for elaborate equipment.

You asked a few posts back about why the Egyptians would continue to use bronze tools to cut rock for a long time span. Well, there is a simple answer: They worked. In fact other than changing to steel as it became generally available the same techniques were used by stone masons right up into the last century. I expect they are still used in many cases.

Bill Gill
C is not the speed of light in a vacuum.
C is the universal speed limit.