I run a Klune-V, which is a planetary type gear box. Like what Sharp said the 4:1 gear sets are what you would want to run with a doubler either in the T-case or the doubler. The Rubicon 241 would be the box to use since it has the right gears, but they don't come cheap.
I love the doubler type set up. For the smaller motor they are a big advantage.
For $550 that is cheap compaired to the Klune. They are $1800+. But everything is new, not used.
There is always a bit of a demand for the doublers.
What where you thinking as far as making one? Taking a 231 and turn the planetary setup into a reduction box?
If you want to make one since you have the machines to do it try one. See what kind of time and materials you would have wrapped up in it.