Each 'frillie' is created as a mated, mirror-image (left and right) of each other. Cut out and attached on each side, one pair at a time. This way, each 'level' on both sides is the same size width and height. Only the bottom 'frillie' is riveted to the collar whirl at this time, and the 'cut-out' fits over the collar allowing the bottom to almost reach the shoulder area. All subsequent frillies are only riveted to the one below it, not directly to the neck-whirls. This allows the 'tower of frillies' to be bent forward towards the temple-cheeks of the snakehead. Only then are they riveted with brackets to the collar whirl. There will be tension on these when the back-of-head scales are attached, so you want to make sure that the frillies are secure.
  I prefer at least two rivets in each 'end' of the 90-degree brackets, and where one frillie overlaps another, to use four or five brackets for greater strength and rigidity.

  This sketch shows the shape of the spline anchor and cheeks. Much of the 'tail' of the head-spline might be cut-off later, and the 'eyebrow' above the cheeks trimmed. It's not too important therefore to retain the exact demensions of the back-of-head spline. Just make the two left-right halves symmetrical, and also the cheeks, Left and Right.

  Use those 90-degree window-frame brackets here, especially important around the 'nose' where the cheeks and head-spline meet. The 'bending' of cheeks imparts tension, you need the nose to remain in place. The 'nose' and the 'tail' are no narrower than 2-inches. The 'tail' can be wider as it will be covered with the back-of-head scales, but keeping it 'less wide' ultimately reduces the overall weight in the finished product. -So, keep it narrow. Two inches wide is adequate.

 Outlined in light blue to show the edges, begin with the bottom frillie, numbered one in this image.
 Rivet each new frillie to the one below it, but NOT to the neck-whirls. We will do that later, using those window-frame brackets, when both towers of frillies are completed.

  The 'head spline unit' is attached with rivets to the bottom back-of-head scale, and the cheeks are ready to be riveted to the top-most front edge of the neck-whirl. With the headpiece affixed to the back-of-head scale by the 'tail', center the 'face' of the Serpent's Head over the upper neck-whirl, and rivet each 'cheek'. This forms an 'arch' from the back-of-head to tip of the nose.
 Align each tower-of-frillies and attach with rivets to neck whirls with window-frame brackets, bent to accomodate each shape required. Some skill and creativity is needed here, -and keep every rivet tight and strong.

 You can now begin measuring and attaching each back-of-head scales, using 90-degree metal window-frame brackets to affix each end of the 'scale' to the appropriate 'frillie', working from the bottom and progressing up, and riveting through each back-of-head scale into that 2-inch wide 'tail spline'. As with every rivet in plastic-to-plastic, use 'backing plates' for maximum strength.