OK Nicky B, here’s an alternative to trying to bend threaded rod perfectly to fit the holes in the Miyata seat base. I had a couple of the devices shown in the photos laying around the workshop. I tend to collect bits and pieces of hardware figuring I just might need it someday. This time and probably this time only, it turned out to be true.
No, but there is no need for such items in a carbon fiber saddle base. A properly sized square hole in a carbon fiber base is sufficiently strong enough to hold the shoulder of the carriage bolt firmly.
Instead of pounding them in with a hammer, I’d suggest sinking them into the plastic of the Miyata seat base just by pulling them through with the bolt. Drill a pilot hole first, and use a small washer on the opposite side of the “T-nut” and just tighten the bolt enough to draw the prongs of the T-nut through the plastic. For added strength (to beef-up the weak plastic) you could use a fender washer on both sides of the plastic seat base (but on the inside, the fender washer has to go above the T-nut… ie; NOT between the plastic and the T-nut).
This would work in a carbon-fiber seat base as well, but you don’t really need it. It would make things all that much stronger if something like a T-nut were integrated in the carbon-fiber matrix from the beginning, but perhaps it would be over-kill…