Here is a sketch: Edit this sketch is the finished dimensions of the baggage area flat pattern. Fabric cutout will be larger for velcro, and for stitching the corners up as described below.
In case my writing is hard to read:
The trapezoid is 36.75" along the seatback, and 30" in the aft part of the bay. It has a height of 18", meaning between the seatback frame and the next frame aft. The sides of the trapezoid end up being about 18 3/8" long. I think I drew two parallel lines 18" apart, drew a centerline, and centered the 30" and 36.75" lines then drew the sides.
All 4 sides have a depth of 10". That works for me - you can adjust as you see fit.
On top of the 10", you need to add at least 1" for the velcro to attach to the airframe. The longerons have a width of 1", the box structure behind the seat is much wider, and the frame in the aft of the baggage area is greater than an inch. So add at least an inch. I used very light 0.75 oz ripstop nylon, so I think I added 2" to the 10" and folded it over double before I sewed on the velcro.
On the sides, you need to add an inch or so to sew up the corners. Anybody who has made a bag or cushion cover will know they need to do this.
Sew your velcro on before you sew up the corners. It's a lot easier. The baggage shape is not very nice until you put the foam posterboard in the bottom. So put velcro on the bottom to stick the posterboard to. You can use something else - luann plywood, aluminum, 1/4" steel plate, granite or whatever. Also consider sewing some velcro onto the front wall of the baggage area. I have mating velcro on my toolkit pouch, my decalin container, and my 1 qt oil container that I sometimes carry. Keeps the heavier stuff forward and upright.
Edit: For the triangular closeout, I also sewed some velcro on the top of the aft wall of the baggage area for the triangle to velcro to. Add that to the velcro list before you sew up the corners.