Cinder blocks are a good way to go.... but not fence boards... they are way to thin and will warp and become useless after a few seasons not to mention heavy trees will put the hurtin' to them quick.
I use the big cinder blocks stacked 3 high with the row bieng 2 deep end to end. I will then replicate that every 4 feet that way in the middle of an 8' span there is solid support. I then use a treated outdoor 2x4 deck on top of the cinders.
Instead of monkey poles I use timbers.... much more impressive looking, very grand, will last forever and are much tougher than any pole out there. Big problem is getting them in place. My timbers were 8 to 10 foot tall and are between 20 and 36" in diameter. Talk about heavy. I had a tree dolly but then had to wrestle them in place by myself. That meant lifting them. Good thing I am not a little person! haha
Pics of the projects....Before I had trees put in place. It looks much different now with everything grown in around the poles.
I like the pond dude, nice koi and it looks like there might be a goldie of two in there.
keep it green,
Harry