Nice little pond!
You can make a small bog filter.
Since you seem to be very good at building with wood....
You can build a smaller box the width of your pond and something like 12 or so inches for the other dimension, similar to a window planter box.
Have it mounted in the back, in place of the waterfall you have. One long side will be hanging over the pond.
Cut a notch out as a waterfall.
Lay a liner in it to hold the water.
Make a distribution pipe from PVC to lay on the bottom of the box. Cut slits in the distribution pipe.
Cover the distribution pipe with pea gravel or river stone.
You feed the pond water through the distribution pipe. It rises up through the gravel and spills over into the pond via the notch you made.
Add plants grown directly in the gravel, no pots, no soil.
The gravel serves as a place for the beneficial bacteria to colonize and the plants remove the excess nutrients in the water.
The result is crystal clear water and almost no maintenance.
Or, you could just use an actual window planter box and if it doesn't have any holes in it you won't need a liner.