outer rubber insertion
Sliding it in is not a workable option as you found and as I found recently. I have a plastic insertion tool which is really just a wide screwdriver like end which is curved.
I inserted a length of the front face of the seal then used the tool to prise in the rear part. I only did this in small sections - about 60 mm at a time and used glycerin sparingly as a lubricant only on the rear facing part of the seal.
The corners - especially the tighter ones take a lot of patience but it does go in. A few swear words here and there seemed to help.
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