I tried lots of under-bumps.   Even stacks of terraced felt that looked like a topographic map.  No matter what I tried, the tail just didn't look right.  Finally, I decided to try just cutting the fur ribbon into the 2-D shape I was trying to make in 3-D.

Success!  Oh look at that luxurious fur tail!

The only issue on the flat shape is that the fur has a direction.  Along the back edge of the curve the fur flows away from the edge and the under-grid to which the fur is attached is exposed.

I carefully curled under that edge of the fun ribbon and sewed the curl in place.

Here's what the back of the tail looks like.  with the one edge curled and sewn.

Here's the tail from the front.

Strangely, just that tiny bit of work makes the tail look 3-D.  The fur seems to grow from behind the fur ribbon and come to the nice foxy point near his little paws.

The next post is going to be the big reveal ... woohoo!