I use one for mostly resale value. Eventually, I sell off every detector I own. It is a selling point with many buyers that you have a cover on the coil. SO for the most part I use one. I would not goop on epoxy and truck bed liner, however, again based on resale needs.
An alternative would be to adhere it with a few dabs of silicone RTV adhesive, evenly spaced around the cover's sides. Just use enough to keep it on.
Or you could put it on in such a way that you seal the cover with it completely. That way no dirt can get between the cover and the coil.
Whatever, you do. be thankful you have a loose cover, in fact. Sometimes, they can be a real challenge to get off if they are tight!