This is a close-up of the cap that has done most of the work in my TC system.  I initially used it alone to provide about .013 uF, and later I cut some of the bottles out to make about .007 uF and used it in parallel with my LDPE cap for about .026uF total.  The construction was ultra-simple and the price was right...about $2 for salt and $2 for the vegetable oil on top.  This cap was a real performer 'till the end -- even if it is kinda ugly!.  Ironically when I replaced it with an extra section of LDPE, the overall spark length only increased about 2".  Apparently RF loss in the cap didn't reduce coil performance as much as I was led to believe.