The back stretcher will be rotated 39 degrees (roughly) from vertical to allow it to be joined at the thick part of the leg and will be wide enough to reach up to support the shell near where the seat turns into the backrest.

Like the front stretcher, I started with a scrap stretcher to verify the fit using the calculated compound angle end cut.  With the angles cut, in order to get the ends to fit flat against the legs, I did need to rotate the stretcher slightly, but I was fine with the final angle.  It was only off a little.  The discrepancy could have been due to being off on the miter saw settings for either the front stretcher or the back stretcher or both.  (Most likely both)

With the scrap stretcher fit verified, I cut a wider version from oak.

The weight on this stretcher will be on the angled edge and there will be forces trying to rotate it.  I thought it best use a flat cut blank to keep the grain roughly vertical on the rotated stretcher.  This should be more resistant to the edge breaking-off than if it were quartersawn.  Whether it was necessary, I'll never know.

I shaped the back stretcher in a similar way to the front, although this one is compressed in length and expanded in width.  It also has a continuous support surface for the underside of the seat.

I confess, I didn't spend as much time noodling over the shape as I did for the front.   It seemed I had fewer options in the compressed space.   But, I did have 1 revision.   I also added the domino locations in the design to aid in layout.

I marked the center of the Domino location on the template, then transferred the marks to one end of the stretcher.

Using the lower straight edge as a reference, I transferred the marks from one end to the other and then extended the marks perpendicular to the ends.   With the front stretcher, I had cut it to shape prior to marking mortise locations, which meant I had lost my reference edge.  Marking the back stretcher now rather than later means that if I'm off a bit when positioning the routing template, the stretcher will still fit properly.

I re-fitted the stretcher blank into place and scribed a line where it met the leg and transferred the Domino locations.  I did this on both sides because I'll not be making a jig for cutting the leg mortises this time.   

With the thickness of the piece and the angle of the bevel, I needed to place the mortise close to the edge to ensure that I didn't blow out the side when cutting it.

It turned out that the mortise needed to be centered 8mm from the inside edge of the bevel, which is shallower than the Domino can cut without a shim.  I used some scraps (what else) of 1/8" birch ply to create a shim, and transferred the Domino marks to it.  This was fixed in place with some double sided tape.  The unmarked scrap is just for extra support for the fence. 

Once cut, the process was repeated on the other end.

For the leg mortises, since the center of the Domino mortise is 1cm from the fence, I marked 2mm beyond the edge line and clamped a fence there.  This will allow the mortise to be cut 8mm from the edge line.

Instead of transferring Domino location marks onto the fence, I used the sight line on the fence to align with the marks on the leg.  It was a little harder to see with the fence folded-up, but it worked.

Once cut, the same thing was repeated on the other leg.

With the mortises cut, I did the usual dance with the template, cutting most of the excess on the bandsaw and then template routing on the router table.

I did not cut the top edge initially.  I wanted to see how much height I had to play with to make the curved bevel that meets the seat.

With the seat set in place, I could see that I had plenty of leeway to rough cut a perpendicular curve along that top edge before creating a bevel.

I used a combination of rasp and sander to create a bevel that fit against the bottom of the seat.   I really need to work on spokeshaving skills.  That ugly mess of fiberglass where a support block had been is not long for this world.

And another look-see at how it's coming together.  The clamps are there only to prevent the legs from splaying further during a test sit.  Feeling comfy.

Next will be more shaping.