@Byteguy,
I had no problems with a slipping cover falling down, but the plastic cover in previous HW versions of CPE was closing the cable duct somewhat tighter, sometimes too tight. However, there are two small plastic noses/locks and you should here a »click« when closing the cover. Those noses/locks should keep the cover in place.
But if you open and close the cover many times and the noses wear off, then probably a part of Butyl Rubber Tape (used for electrical isolation of outdoor connectors such as N-type or RP-SMA antenna connectors, SAT F type connectors etc.) can help to fix the cover. This tape is weatherproof and resists temperatures from -40º C to 120º C.
It's also useful to cover plugs or other electrical parts and I also use it for isolating cable ends/connectors on my motorbike. Since it is self-fusing I even used it to isolate tubes of an extractor hood. Note that the tape is not adhesive, so if you don't need it for other isolation tasks, maybe you can ask an electrician in your region to give you a tiny piece of it, they should have such tape:
What I am missing from all CPEs (510/210) is the waterproof rubber insert which is included in WBS510/210 (at least it came with early HW versions). The Butyl tape doesn't help much here, because it's too thin for the gap between the duct and the cover. Over the time I have much more tiny spiders in the duct of CPEs than in those of WBS. :-)