Thanks. The Japanese part of the garden is a work in progress. It was developed from farm yard junk dump basically. This was the state of play 2 years in, as of last year some time, just to give a style idea of what I am dealing with,
Hopefully photos will appear here. One looks down towards the small Barn that I am remodelling inside currently. The side you can see will be ripped out and fully glazed when I get time. The "canal" is about 35 metres long (those boards are very long and were a nightmare to deal with as I had to be in the water to fix them) and links a couple of ponds (not this Koi pond), one of which is a wildlife pond with common and great crested newts that appeared from nowhere last year.
The Bonsai to go on the lid has yet to be purchased, but what I have in mind is a grove of pines in a shallow ceramic tray. It's not especially heavy.
My thought was use either Buffalo board, Ekoboard (anyone used this?) or maybe marine ply painted black, edge it with green oak, and lay strips of machined oak roofing lathes to create spaced oak decking to mimic the ultra heavy duty decking adjacent to this Koi pond. I don't want it to look twee, and it needs not to detract from the Japanese style of this bit of the garden. I have a lot of oak roof lathes in my yard and it wont take two ticks to run them through the PT. However, it does not feel all that creative.
The area gets wet constantly. All of my electrics are waterproof so that is no issue really, but I would prefer to keep the filter area dry or dryish.