I'm using the solid surface counter-top material Corian since it is not porous and it is relatively strong and a 1" thick slab will not deflect / fail (crack shatter) when under a vacuum. You can glue it together and the epoxy glue welds it so that it is strong as heck.
My first attempt at making an end for the kiln was a failure. I had to go back to the drawing board and learn how to use a new tool for the vertical mill --- the 360 degree table. It is so versatile, I wanted to do an instructable on it.
Step 1: Rubber and epoxy failed -- Solution use a O-ring
I thought that this would work like a charm! At first glance, it seemed to work. Unfortunately, the vacuum pump that I'm using is so powerful, it is pulling the rubber from the corian when under 28" Hg.
I decided that I needed to go a different route -- I needed to make a giant O-Ring and route a channel 1/2" into the corian to mate with the 12" PVC pipe.
This was not a job for a hand-held router. The solution is to use a vertical mill and the attachment for the mill that allows you to make a perfect circle -- the 360 table!