post box treatment

I have built a post box out of good quality ply, this will be fitted to a post going into the ground. I am looking for advice for the best weather treatment for it. There seems to be so many way i.e. stain + varnish, weatherproofing stain, garden furniture paint, normal primer+undercoat+paint, I assume I will need to coat the inside of the box aswell as the outside and I don't plan to put a covering on the pitched roof so the treatment needs to be good, I am not sure what to use/which system to use.
Any advice please, thanks

Sadolin superdec . When it weathers just re paint it. Inside the box it will be fine too. 2 coats onto bare wood

An oil based gloss would be fine aswell

thanks guys