An update to this:
The elastic straps that I bought were apparently old and the rubber bands eventually broke. I did find some new bands that are EPDM and Ozone and UV resistant so should last a while.
Concerning the slide covers. I did find a few parts from a W108 sedan and a couple of those fit my later W111. None of the W108 driver's side work as that is completely different from a W111 car. The passenger seems to be similar to a later W111. So from the (2) plastic parts I was able to make a complete set for my '68 W111. See attached. The originals are plastic and I made a paper template from those. My new ones are 22 gauge galvanized sheet metal. From here, I will cover them in leather and know one will know. The parts on the left are for the outer rail and on the right, the inner rail. The parts on the driver and passenger are just mirror images of each other.
Not sure if these will fit the earlier W111 220SE and 250SE seats. They should fit the all 280SE seats however. They should fit the W108 passenger, but not the driver's side. I would be curious if someone with an earlier W111 car could determine if these would work? I can scan the templates and send them. And if anyone would like to attempt this themselves, let me know and I will send the templates.