When I was in grad school I made one out of a circular fluorescent tube, meant for a household lamp fixture. I think it cost me like $8. Of course in those predigital days I had to use wratten filters to correct the color. I was shooting space filling molecular models that needed to look like they were floating, so the shadows had to fall behind the object. Again this was pre digital....no 3D software. Back then we had to improvise!
