You can download this polar opening topographic program created by Scott Macklin (an associate of Rodney Cluff) to simulate different possible sizes and placements of the hollow Earth's north polar opening. The program is relatively sophisticated in its simplicity as you can take into account the thickness of the Earth's crust at the location of the opening among a variety of other variables. In the image displayed above, the crust was assumed to be 600 miles thick and the distance from rim to rim is 650 miles (arrived at using calculations with the polar and equatorial diameters of the earth). You can compare this with what the opening might look like if the crust were only 400 miles thick at the pole. This program provides a fantastic visualization tool for those intent on better understanding the nature of our mother Earth, or could be utilized to aid in charting a course if an adventurous individual was intent on actually sailing to the inner earth much like our friend Olaf Jansen.
Download link here.