Here are some simple, D cross-sectional reamers I've made for flutes in various keys.
For each reamer you start by taking a lot of bore diameter measurements to produce a map of the bore.
This requires some specialist tools. Then you start with a cylindrical piece of tool steel that is larger than
that and you turn it down to a stepped conical shape that matches the measurements. Then you carefully
smooth the steps by filing and polishing. Finally, you have to cut out a section of the reamer to produce a
cutting edge, without losing the overall dimensions. All of this takes a long time and you have to be meticulous.
If you over cut anything then its back to the beginning (or suicide!).
Obviously, I'm skipping a lot of important details here, but this is to give you an idea.

The picture below is what it looks like before you cut it to produce a cutting edge.

The problem with D section reamers is that the cutting edge isn't very sharp, so they don't cut very easily.
I have addressed this by cutting a concave surface into some of my reamers. This makes them cut much more
easily. Here is one being cut on the milling machine.

If you want a really nice cutting reamer you can cut multiple cutting edges, like this. This requires more
skill in milling. This one belongs to Geoffrey Ellis.
