One cannot just cast up an insulator. The glass to metal seal is critical and "graded" seals are common and complicated. (uranium glass to special borosilicates, etc) The metal must be specially prepared as well. (nitrite treatments or other heroics.)
In general, you want a very small diameter metal rod feeding through the insulator,less than 1/4 inch in diameter. Then there is the complicated mounting flange to glass seal, which is even more difficult to fabricate.
By this time, commecial products are looking better and cheaper all the time.
Richard Hull
Created on Friday, March 02, 2001 4:57 PM EDT by Richard Hull