Probability is greatest on the side of an aircraft illuminated by a low-lying sun. The image you saw in your binoculars looks suspiciously like coma aberration for small angle source of light (like a reflective aircraft body). Coma is very likely when you don't look exactly along the optic axis of the binoculars. Since many binoculars have large exit pupils, it is very easy to create this aberration. Likewise, coma aberrations may exist not only in the binoculars, but in the eye itself. You just don't notice it except under special conditions. In other words, you possible saw an image that was distorted by coma and the actual object had a very different appearance. Here is a crude example of coma, but it can also looks very similar to your drawing.
There is no way that altitude can be determined, so to claim it is in the stratosphere is moot. Since you have no idea what the dimensions are, the visual angle of anything you see tells you nothing about true distance. Naive people viewing the planet Venus at sunset or sunrise often believe it to be a flying vehicle near the ground and close to the viewer. Your brain often makes poor decisions about images when there is little information available.