Imagine trying to change a bicycle tire if the rim of the wheel were flexible, and falls off, and that will give you an idea what a pain in the neck this job is. You need either a friendly octopus to help you, or lots of clamps, so that once you've seated the clamp in the top part of the seal, you can apply lateral pressure to stop it pulling out (and bringing the seal with it), when you apply force to stretch the spring over the bottom part. You can fit about 2/3 of the circumference without difficulty, and secure it with three or four clamps, then push the spring down with your hand, and use a screw driver like a tire lever to pop the clamp over the rim of the seal and into the groove.