Basic Cobra Trooper shirt pattern

Is there a pattern anywhere for the basic Cobra Trooper shirt flap or is it pretty much a “measure and cut” type thing?

The field manual suggests buying a second shirt 2 sizes up from what you wear:

Embroidered Chest Flap
For Cobra Troopers, a large red Cobra sigil should be worn on the center of the chest. The preferred technique for this modification requires the purchase of an additional, larger, Propper Dark Navy shirt (any style), at least two sizes larger than your normal size.
Use the back of the larger shirt to make a flap which should then be embroidered with a red Cobra sigil. Check with the Finest Command Staff for recommended embroiderers. Once the embroidery is done, sew one side of the flap to the side seam of your primary uniform shirt. To access the shirt buttons, use Velcro to attach the other side of the flap.

Basically you are ripping the stitches out of the rear panel of the oversized shirt which should be close to the appropriate shape of what you need.

Yea you kinda want to fit it to yourself. Once you cut out the material you will use, try to mark/size it to where it will sit and basically sew the cut sides to fit the attachment velcro or whatever you use.