Electrical-Insulating Female Threaded Standoff. 3 inches long by 2.5 inches diameter. 3/8-16 thread both sides. Removed from a 600 volt AC system but I believe rated much higher. Two available @ $8 each OBO.
Also have 6 each 2 inch long by 2 inch diameter Electrical-Insulating Female Threaded Standoff with 1/2-13 threads both ends. All have brass 3 inch long stud installed in one end. Stud can be removed with some effort as they seem to have sealed threads. $8 each OBO.