If I understand this right, I see two options:
1. An outdoor rated receptacle (common in the US), not C8 of course
2. Make an adpater from this
**broken link removed**, your mains plug, a cord, a cord gland, and a watertight box.
Even use a standard IEC power cord like used on computers and cut the end off.
The only real comment I would have is to place the C8 receptacle not facing up.
So really, just put a watertight box on the end of an IEC power cord with the C8 connecter mounted and siliconed in place. Face the C8 receptical downward and you won't have any water problems. Everything else is water resistant.
An example case
https://www.polycase.com/surface-mount-wc-series They also sell the glands on the product page.
Use dielectric grease on the plugs and sockets.
What mains plug do you use?
US version of an outdoor outlet which is usually protected by a GFCI (ground Fault Circuit interrupter). Your XMAS lights should be, We can get GFCI cords such as this one:
**broken link removed** and seach for G2015194
SO, just add a cord gland, watertight box and a male C8 and your there, I think plus a little silicone and dielectric grease.