The plate is aluminum, I had the guy shape it so I could bolt it down directly to the plate. The plate itself is bolted to 3 steel clamps that clamp directly to the frame. These are made of heavy-gauge metal. Feels Pretty solid to me but I'm no longer saying stuff like "if it fails, I'll eat my shirt".
Just another picture with the fuel cell on top of the bracket. When it is fully assembled, the only visible difference from my old one is when you look at it from behind, you see that it's made of aluminum instead of plywood. One sucky thing is that he used bolts that are 9/16 and 7/16 instead of metric. Now I have to carry more tools... On the plus side, there is now about half a centimeter more clearance where the valve for the fuel cell is located and I can now open and close it while wearing gloves 'on the fly'. I used to have to remove one glove while riding to turn the valve on or off.
I've had the chance to try it out last weekend for a short but bumpy 250 km ride and it feels solid. I guess time will tell if it holds...