A bowl suspended by forty feet of string?

Why not four feet? Or four hundred feet of string?

The longer the string, the better: a longer string can take more winding up, and so impart more spin to the bowl for a longer period of time. However, to prevent the wind from blowing the apparatus around, it needs to be inside a building or in a shaft in the earth, or similar.

Here's Scientific American on the shaft at Teotihuacan (it's near the center of this post's image, off a little towards the lower right.)

The tunnel itself was discovered when a heavy rainstorm exposed a shaft that led to a spot about halfway down its length. The shaft’s purpose remains a mystery but scientists believe the tunnel had a ceremonial purpose, and it is possible that the shaft was used for astronomical purposes.

That shaft was 14 meters (about 40 feet) deep. A number of other astronomical holes or shafts of about the same depth seem to have existed.  El Caracol looks to be about that height. I haven't found any location with a much deeper shaft, so 40 feet seems the best guess.

Assuming the user of the hypothetical telescope will be situated near the focal length of the telescope, then the minimum focal length is .2 meters (with the viewer near the bowl,) while the maximum focal length would be approximately the length of the string (with the viewer near the suspension point of the apparatus.)

In the long focal length case, the bowl would rotate at around sqrt(447/14) = 5.6 revolutions per minute.