Are these available in sizes 20mm and above or must one make there own?

Not hard to make, just cut the seat in two. You can sand or file the remaining pieces flat. If you have a stationairy belt sander it will make quick work of the job.

I do not think you can get size 20 seats in pre formed skeletons.

I make a lot of skeleton seat assemblies for spinning rods. Unless something has changed, the Fujis can be had in all colors in the 17mm seat, only the silver in the 18mm seat, and nothing in the 20mm seat.

As Rob mentioned, all it takes is a band-saw and some sandpaper to make it all perfect. Just cut the stationary hood off. Then, position the hood/nut directly over the threads, and cut on either side of the hood/nut. Now, what you do is to take the pieces to a disc sander (belt sander or plain old sanding block would work) and use the edge of the hood as a straight line to sand everything square.

If you're going to make a lot of skeleton seats, it's best to find or buy another reel seat to use as a sacrificial seat. Take the hood/nut off of it, then use that "junk" hood as your sanding guide. That way, you don't mess up your good hood by accidentally getting it too close to the sanding disc. I have a collection of 16, 17, 18, and 20 mm "junk" hoods that I use as sanding guides.

Here is another way to make the seats.
If you have a lathe, you can make up an arbor that is a tight fit on the skeletal seat.
Then, simply flip on your lathe and either use a parting tool or abrasive cut off wheel to make the skeletal seat with clean and straight edges.

With the right tool, it works very slick and leaves a very clean and straight edge.

Take care
Roger

I have never used one or made one so hears the dumb question of the day. Do you use it without say an "insert" or just the front and threaded part and let the reel foot span the blank? Kind of a blank thru is what I'm saying!

John,
The conventional way to use the skeletal reel seat is to use an insert of some material to replace the normal plastic molded part of the reel seat.
For example, St. Croix has marketed their Avid series of rods with a skeletal reel seat using a nice cork insert between the front and back part of the reel seat.
Others commonly use various kinds of wood that have been turned down to the correct dimensions.

Recently, some folks are using the reel seat without any insert and are having the reel seat span the empty space between the front and back reel seat portions.
I have held and fished with such a rod, and intensly dislike it. Very very very uncomfortable to hold for long periods of time. I would much rather have something filling the palm of my hand for an overall better comfort and grip.

Take care
Roger

Various vendors listed on the left of this page carry various kinds and types of the commonly sized skeletal reel seats.

Thanks for the suggestion and I was thinking that with the reel spanning the blank it would be uncomfortable. I might just try a test fit to see what it feels like. Thanks again.