I forgot to take a photo of the setup for drawing the arc, but basically I bowed a piece of plywood to the correct vertical distance at the center point of the arc and traced the curve with a pencil. Cut it out roughly on the bandsaw, and smoothed it out with sandpaper.
The mortises will be done with my benchtop mortiser, but before I drive out to my brother's shop I need to spend some time marking out where the mortises will be.
Step 1: Mark the height of the mortise using the actual tenon as a guide.
Extra Step: If you screwed up one of the tenons by cutting on the wrong side of the line, like I did, and it's a little smaller than the other one, make sure you mark it clearly and keep track of it the entire time. It's not a mistake if you hide it inside a perfectly fitting mortise. :)
(perfect, my best fitting through-mortise & tenon yet)
I used my brother's router table with a straight bit to make the double-stopped rabbets on the stiles. I'll chisel out the corners by hand.