If you run the seat all the way back on the rails, it will bend. I've demonstrated this over and over. Push it all the way forward, and your problems will go away.
That "proper" seat guts is for a B67 or B130 saddle, with two rails on each side. For a single rail saddle, it won't work even as well as a normal cheap seat guts.
The best I can tell you is to make, or have a machinist make for you, a chunky clamp that attaches to your seat post, that holds another seat post behind it. Or else have someone make you a thick chromoly layback seatpost with a reinforcing gusset.
Don't bother with any seatpost in 22.2mm diameter; none of those will work for you. When I need one that size, I machine it from a solid bar of steel or very high strength aluminum, like 2024-T3 or 7075-T6.
Another thing you can do is to stand up when you know you're going to hit a bump.
This is to express my gratitude to Justin of Grin Technologies for his extraordinary measures to save this forum for the benefit of all.