I've flown (& even made several myself) kites out of a single piece, (icarex p-31 is the only stuff to use) skin of fabric,.... in fact you can "no-sew" them and make a fantastic low wind device! You will use 9460 bonding tape from 3M co. ONLY as the adhesive and "bond" the seams or edge bindings with a teflon-footed craft iron to fuse the adhesive.
Never fear that the adhesive will let go either, the fabric next to it will fail first. 9460 is made to bond two pieces of metal together. I've flown this style of construction for literally thousands of hours with no failure.
Flat is the only way to make the sail, unless you want to add some curvature built into the leading edge (still it's only in 2 dimensions, no depth necessary or even desirable)
The stack pix is a twelve pack of 42 inch leading edge mini-ryvs, no sew construction, single train lines running thru the whole stack and affixed to the bridle instead of the end-caps, no knots or elastic, instead made with automotive oh-rings and tiny belt-loops.
The expert on this no-sew construction method is Harold Ames (KiteSquid on the builder forums)