[Stick486]

cut a floating tenon to fit and slice or cut off the length the tenon to your happiness...
treat this as an inlay such as a butterfly/bowtie..
and extreme jig to do this would be a Leigh FMT Pro...
you could make your own jig...

to make a floating tenon it takes nothing more than a piece of stock to size and a bullnose or half-round bit

 

[Stick486]

no bullnose or half round???
use a 3/8'' round over and your RT...

 

[Stick486]

here's an image of a loose/floating tenon...
you can do the tenon in end or long grain...