I just recently replaced the broken base on my KH saddle with a carbon fiber base. I was ready to file square holes and use carriage bolts, when I came up with another idea. I took (2) 3/4" 1/4-20 bolts and a piece of 3/16" steel rod.
I then brazed the steel rod to the 2 bolts. I made 3 of these. 1 for the back bumper and 2 for the front handle.
This assembly holds the bolts in place and keeps them from spinning when you put tighten the nuts to hold the handle and bumper in place.
The right bolt looks crooked, it’s just the angle that it was photographed.
I used (4) 1/4-20 bolts and nylock nuts to hold the seat post in place. Instead of using a rubber gasket between the seat post and the cf base, I cut a rectangle from the side of a plastic bucket. The plastic gasket offers protection between the base and the post, but doesn’t compress like rubber does.