I have not had this problem (yet). I assume there is little to no room behind the seal. Is it possible to find a washer the size of the ID of the seal (when the spring is in it), stretch the spring over the washer, push the assembly through the hole diagonally (maybe tape it with a few retrievable strands of masking tape), then pop the spring up in its home position?

Time to get creative.

Hey, didn't you buy a USB borescope? Looks like there's a view hole behind it...

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