There is a metal junction box sticking up 6" out of the ground mounted to the top of a piece of conduit. There is a rubber gasket and metal cover screwed to the box. Inside the box is simply six wires, 2 black, 2 white, 2 ground, each pair wire nutted together. I want to connect a 120V 15A receptacle to the wires, put the entire receptacle inside the box, and close the lid, making sure no screws can be shorted. Is this acceptable?
Outlets have to be secured to the junction box feeding them. You can put an outlet there but since it's outdoors, it would have to be GFCI protected, preferably at the main panel. If not there, then a GFCI outlet would need to be installed in the box. You could then put a locking cover over the outlet similar to the one shown below. You'd want to check that the existing lighting load and future load of the outlet won't exceed the breaker rating. You might need a box extender depending on what exactly is in the field.
That is called a bell box and yes you can add a receptacle to it if not on a GFCI circuit you could use a WR rated GFCI with a extra duty or in use cover and this would be code compliant. And yes it needs to also be tamper resistant. I use these boxes as junction boxes all the time with a blank cover but when adding the receptacle a special cover is required to keep the rain off or out of the receptacle.
Assuming your circuit is suitable for adding a receptacle in-line (e.g. not a dedicated circuit for a pump, or some type of switched circuit if you don't want the plug going on/off), you could install a receptacle in the box. To make that proper, you would need to install a 20A GFCI receptacle with a weatherproof "in-use" cover that allows you to use the plug while the box is closed (the flat types aren't up to code anymore although they are still sold). They have metal ones with holes for a padlock if you think you might have issues with random people using it.