I was thinking along the same line as Rob but I can see how that could be difficult without having to clean up the face a bit. Not impossible but difficult.
Since you already have your form blank glued up, it's a bit late for that but you could start over if you wanted.
While reading the answers, I came up with an idea.
This will require making two templates. First cut and refine a template outlining one side of the form. Use that templat and a 3/8" router bit to cut a second template giving you the two sides needed (male and female) much the same as Rob described.
Now you can use the pair of templates to refine your form.
If you have a spindle sander, set it up like a router with a bushing. How you do that is up to you but it shouldn't be too difficult. Lay out the shape of the form (3/8" wide) and cut it down the center on the bandsaw. Fasten the template halves to the form just back from the edge and sand it to the final shape with the spindle sander and guide bushing. It won't matter if the template isn't positioned perfectly because you will always end up with the same profile.
You could do this to make several identical forms so that you can bend more than one splat and leave them in the forms.
This is just an idea I just came up with so it may or may not work. It won't work if you don't have a spindle sander.
Has anyone tried this?
Is there a downside that Im not thinking of?
Another thought. Any industrial forms I have seen have alignment pins. Make your form longer than you need it and install dowels as alignment pins in the ends. That should help a lot at glue up time because it gives you one less thing to worry about sliding around.
J.P. Rap Mount Hope Ont.
Carpe Ductum (Seize The Tape)
"In this world, you must be oh so smart, or oh so pleasant." Well, for years I was smart. I recommend pleasant. Elwood P. Dowd