How to cover old Red Dot outdoor outlet for constant use?
I think mark f has already answered your question well.  I just wanted to add some photos from a similar project I did last year in case these are helpful to you.
I installed a weatherproof cover and box to replace the old ones that supplied my above-ground pool.  Fair warning: don't do this at your own pool because my installation was deficient in several ways; but I bought the house with some problems already existing, and I just needed to make a repair and add GFCI for my own comfort; my city didn't care.
Anyway, first picture is what the box looked like with a couple duplex receptacles in it.  Hopefully, you won't need a new box; but if you do, they come with plenty of plugs and conduit bushings.

I prepared the wiring in my garage:

Then I have the parts that came with the cover; that's the cover itself plus a gasket that makes it watertight.

Here's what the box looks like with the gasket fitted:

And now with the cover attached using four mounting screws:

What it looks like when closed:

Finished, with a cord plugged in:

I hope this helps!