If I wanted to do this, I wouldn't want to mess with seam sealing or risk messing up the pants if I did a bad job. I would look inside the pants. How high up does the gaiter attach and can I sew a pocket onto it that will be out of the way? Is there any fabric from pockets that hangs down lower and I can attach a goggle pocket to? I'd also look inside my jacket(s) for a space where the lens case matches the contour of my body and I could install a pocket to hold it.
Up high the gaiter is a good idea. I put the lens case against the upper boot, front and back, and it's about the same shape. My only concern is the lower part of the pants, in the calf area, might become too tight with the thickness of the boot plus the lens case. And the rattling sound of the lens case bumping against the boot could be an issue. But the boot does provide extra protection versus just resting the lens against bare flesh during a fall that I might consider this option, if my lower ski pant area have enough volume to accommodate the boot plus lens case.
I've tried resting the lens case against the torso area to trial putting the lens in a jacket pocket and the shape just doesn't fit. It can potentially pierce or bruise the ribs in case of a fall.
The front upper thigh area seems to be the most natural place for the lens case to rest against. It's the same shape as a lens case and it's fleshy so in case of a fall, there are no bones or tendons or ligaments that can crush. It's just muscle that will bruise but that injury is potentially less severe than other parts of the leg. Also, most falling actions during skiing are sideways, so the lens case won't get crushed between the snow/ice on the ground and the front part of the thigh. And finally, it's easier to access vs fiddling with a pocket that's in the inside of the lower pant in the gaiter.