...and a free socket, not mountable socket of the style in that catalogue image posted by Paddler1. But the plug on the cable feeding the lights will presumably still live outside when unplugged, and that doesn't have a weatherproof cover, so storing that in a dry box would be a good idea just to stop the rain filling it up and causing the contact pins or insulating seals to degrade. Note that 60309 plug (wow, they haven't been BS4343 for donkey's years!) can still be plugged in to an industrial socket even if it's a bit damp, as worrying as that may sound, because an IP66 socket provides the necessary insulating space between conducting parts, so long as you remove as much surface water as practicable (i.e. you don't need to get the hair dryer on it first!). Plug it together before plugging in the indoor plugto energize it.
This method you suggest sounds like a good solution in your instance, since you are familiar with using them and already thinking of the importance of keeping the exposed end dry, and a plug & socket pair won't break the bank!