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If you want to make a real black hole then you need to take some mass M and squeeze it inside it's Schwartzschild radius rs where

rs = (2GM)/c2

G = universal constant of gravitation, approx 6.6x10-11m3kg -1s-2
c = speed of light, approx= 3x108ms-1

Or else you could do as I did yesterday and buy a very large rubber sheet. If you are in New York there is a rubber shop on Canal Street and they sell sheets of rubber by the yard !(which is a short meter). Punch a small hole in the rubber sheet, get a plastic disk, punch a hole through the disk and thread a string around the disk and through the hole of the sheet. Have about eight children holding the sheet taught and another child under the sheet pulling down on the string attached to the disk. This forms a very steep depression in the sheet with vertical sides, like the potential of a black hole in space time.

Don't fall in !

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