First attempt at making the bracket was a failure. I transferred the hole locations poorly. I wanted to save as much material as possible in strength so chose to start again instead of increasing sizes or rounding things out. That's why I bought an 18" long bar to work with!
Second attempt is going better. I have the three mounting holes transferred and countersunk and test mounted the full bar without cutting that yet. I have the mount point for the stem "rod" started. For that I went with a really big fastener. I don't have a bit big enough to countersink that so will need to shave about a mm off the countsunk head on the outside of the fastener. The size of the bar and fasteners are total overkill, but, that's a good think in this area!
Next step is to setup the mounting rod for the stem to clamp to. Once that's good, I'll work on cutting the mounting plate from the large bar and rounding off things nicely.
The extra thickness of the mounting plate versus the original hollow alum spline assembly is reassuring even with the fastener holes. As that piece also had the same size mounting holes obviously.
It's looking that this will work for sure. I just need to measure thrice and cut once to keep it clean and solid.
Overall due to the drop in stack on that stem, plus the extra stack up in the thicker mounting plate.........it'll come out to lowering stack by about 1/4". That's fine. I have more stack spacers for the base bar risers.