My perfect binder would have a multilayer construction.
The inner layer would be sort of like an underwire bra construction: relatively flat, pec-shaped cups with rigid 'underwire' material to hold them tight against the sternum, attached to a relatively narrow, adjustable band, with a racerback shape in the back. The cups would be constructed of non-stretchy material, while the band and straps would have only a slight stretch.
The second layer would be in the shape of a short (ribcage-length) sleeveless muscle shirt with a deep V-neck; most of it would be made out of an only slightly stretchy material, but this segment would be cut deep under the arms down to the band of the inner binder and the gap would be filled with a softer stretchy non-armpit-bulge-causing material that would reach all the way up to the armpit. This middle layer would be attached to the 'underwire' portion of the inner binder in the front, but independently adjustable with velcro on one side.
Then the outer layer would be a soft, stretchy smooth nylon that wouldn't cling to shirts and would smooth out lines from the inner layers; this would also be attached to the rigid front portion, but would otherwise be seamless. The outer part could be either half-length to match the inner part or full-length to smooth out the line at the bottom of the binder.
The end result should be, in theory, a male-shaped chest with separate pecs and no horribly-visible binder lines that could be worn comfortably under a T-shirt.