The problem with the standard W124 speaker sizes is that in the main they are NLA. IRRC the rear speakers in the cabrio are mounted on the panel below the rear side window so there may be a clearance issue at the speaker rear with any replacement ? RAINBOW used to be the after market maker of choice but can only offer substitutes now
Mercedes Benz - Rainbow Car Audio
I am unfamiliar with any plastic mountings for the side mounted speakers but they certainly existed for the rear parcel shelf speakers.
This is an area where many start to think "out of the box " Speaker replacement is often dictated by the stamped cut outs in body pressings/location. By changing speaker location to surface mounted door pods, underseat sound bars etc = substitute locations-- a greater choice of speaker replacements opens up. Unfortunately the model is now so old that many car audio suppliers don't cater specifically for your model now- which brings us to fabrication/modification---a little bit of metal work on speaker cut outs coupled with the judiciouse use of MDF baffles / mounts may yield results.With those below rear window mounts I fear your choice of rear speakers may be dictated by rear clearance if using the orginal locations.
W124 speakers
Hopefully someone who has done a rear speaker upgrade on a w124 CABRIOLET recently can chip in with some advice