That freewheel adapter, #6702199, is for 15 millimeter shaft. They also have #6702200 which they show to be for 5/8 inch shaft. There may have been some confusion in the industry about these Sun part numbers over the years, so you may want to specify the shaft size when ordering the Sun brand. I'm pretty sure I bought them for 3/4 inch shaft from J&B Importers some years ago, but the number escapes me. I want to say 6702200, but my own notes contradict me. Just be aware that confusion is possible. I once ordered a handful from some bicycle-related company and received a random mix of two sizes. Staton seems to have a better grip on things.
Until recent years these adapters were hard enough to find that I had 100 of them, in 3/4 inch size, made by a machine shop. That batch did not last long! Now, between Staton and others, they are readily available, and Staton has them all the way up to 1 inch shaft size.
Atomic Zombie also has some such parts, including freewheel adapters with disc brake flange -- kind'a like Jason's clever part.
If you want something really unique, I could refer you to the machine shop I used. They are accustomed to fancy bicycle parts.
I also have a professional blueprint for a basic freewheel adapter, which I can post if you want to see it. It is no magic -- all you really need is the thread, which is 1.375 inch x 24 threads per inch.
If anybody wants a recumbent seat, with a sturdy backrest, and very adjustable, you might consider the one on the EZ-1 recumbent bicycle -- part number 86724, if my notes are correct. They cost close to $150.-, but we love them in Kinetic Sculpture Racing. I get them from A-P Bicycles in Lake Placid, Florida.