If you look in the depression in this rock where the waters is, you can see faint lines that are all interconnected. I have added red arrows to show you where these lines are. These lines resemble the line in the arrow rock. I have made a plug out of balsa wood which will also help you to see the lines in the block. I modeled these lines very closely based on what they look like on this block.
I was told by a professor that there are boxes with crosses on them which I did see on this block but the lines on the base in question seem to join one another as if they were pieces of a puzzle These could be left over from pieces of wood that were used, all glued together to make up this shape.
Unfortunately I did not get to see what the rest of the pattern looked like due to the water, I should have scooped the water out with my hands, let it dry, and gone back at the end of the day to get more pictures. I need to go back someday any way, so I will have to do that next time.
If you look at the two blocks above the block with the water in it, you can also see slight depressions in them as if this cement was poured onto other blocks that were not quite even, leaving these molded impressions in it. It could also be some blocks that were set on to this cement that then slightly sunk into them before it cured. I would think this was poured onto the blocks instead of the other way around.
Here is a small version of this stone I made to show how this could have been made. I matched the lines of the stone to those from the Kantatallita stone so you can see the same pattern.
Here is the underside of the cavity showing the balsa plug still in place. At this point this wood can simply be pried out.
Here is the finished part. The balsa has been removed and you can see the imprint of all the individual pieces of wood that were glued together. I put arrows on it so you can compare it to the original stone.
As you can see this process doesn't need special tools, metal molds etc. They would would have been able to do it with what they had available at the time.
The question is did they know how to make a stone like slurry that has fooled us to this day that these are real rocks?