Here you’ll see a picture of the bottom form and the caul to fit on top. The caul is made from door-skin ply and the parts on the edges were glued on and then it was placed upside down on a flat surface and the other (more elaborate) part of the form placed upside down on top. The weight of the heavier part of the form pressed it down and I scribed the angles on the ends of the wood edge parts and then planed those with a #5 until I had even contact of the center of the caul and the edges.
This is hard to explain and won’t make sense to you until you want to do this sort of thing and read this post again.
You could skip the fitting of the upper caul if you used a vacuum bag, but since I have a bookbinding press I have another option. Plain old clamps would work as well of course, but lacking a press or vacuum bag clamping to the center of the seat could be a problem. Since it will be upholstered it really is not that crucial, but I’d rather make the jig right the first time and be able to use it on many chairs in the future.
The second picture is the two 1/4” plywood seat laminate layers sandwiched between the form and the caul.
The last picture is of the whole shebang squashed in the press.
I didn’t put much glue close to the edges because I didn’t want it squeezing out and making a mess and the finished part will be cut down and shaped anyway.