A couple years ago I made an HP using 1 1/2" round steel tubing with a steel sphere slid over it. The cabochons are 1 1/2" diameter so would not fit inside the tubing. I found a flavored lip balm container that was just the right size at about 1 3/8" and used the top lid of the container for my HP lens. It's not a hemisphere and has a flat spot on the top (so you can flip the top over and place it on the table without it rolling away).

Then I siliconed a piece of milk jug plastic to the inside edges to diffuse the light. When lit up the flat spot kind of focuses the light and actually looks pretty cool. All the pics I have of it lit up doesn't really show the detail very well. The light washes it out.