In answer to your question Colin,
The 1.2mm Ali frame in the pic will be bent and clamped round the profile I made to define the shape/size initially. The two side flaps fit into slots in the hull.
0.5mm plasticard backed with thin card can be curved and clamped round the outer face and the cutouts trimmed back. The card compensates for the fizzy water bottle clear plastic.
This is then removed.The inner face of the Ali and the outer face of the plasticard are sprayed including topcoat.Assemble with clear plastic piece in the middle of the sandwich, clamp together and then trim outer edges once done. Paint edge and touch up odds 'n sods.
Sounds good in theory dunnit!
To be honest Gary, I think your method is more elegant but I'm very wary about how light a touch on the selotape I can make!