Here are a few photos and a crude drawing of my home brew, 450 ohm window line center insulator (feed line attach point).  It is made from 1 inch (inside diameter), schedule 40, PVC  plumbing pipe. You will see I have black Dacron rope attached to the middle of it, running to the peak of my roof. That tie down point is optional, but helps support the antenna and provide strain relief. It is good to do this if it is convenient to your particular set up.

This insulator is nearly indestructible and will handle legal limit plus. It is important to make a good, solid, soldered connection between the feed line wires and the antenna wires. I use several black nylon electrical tie strips to attach the ladder line to the insulator.