THIS or similar!!I feel for you man! I'm highly allergic to all forms of these plants and agree that Tecnu and a hot shower works well to tame it. I typically require a visit to my doctor when I contact it.
When at home have a HAIR DRYER handy. Hear me out on this: The hottest shower you can stand gives relief from itching because it draws out the histamines causing the itch. It feels amazing. Multiply that feeling of relief by 100 when using a hairdryer on your rash. Low heat setting, as close to your skin as you can stand it, move in and out with the proximity to your rash and be careful not to burn yourself of course! When you have severe itching and it's driving you crazy this gives a couple hours of relief. Great before bed if the rash is affecting your sleep.
It may sound unconventional but I use immersion in fairly hot water - don't burn yourself, but definitely as hot as you can stand for an exposure of several minutes without injury. My understanding is the heat causes a histamine dump, you will notice that quickly occurring on exposure to the heat. It is insanely intense sensation for a brief time, almost pleasurable(?). Can give hours of relief that surpasses any topical treatment I've tried and I've wrestled with some bad cases of poison ivy.