Bukstar,
Your setup looks a bit ugly but I do say this as constructive criticism. If your determined to keep that huge ugly filter tub above, I would mostly fill it with water so it can add to overall total volume of your system. Does it have to go above the main tub? If space is limited then I feel you can do much better aesthetically and technically IMO. Keeping to the DIY theme on a budget since I sense thats what your keen to do I have some suggestions for you:
If it were my setup I would remove the top tub totally and replace it with a vertical 4 inch or wider PVC storm pipe as a tower trickle filter. Perhaps drop the powerhead into a large plastic flowerpot filled with 6-8mm pebbles. This will create a biological & mechanical prefilter which is still easy to remove and clean. (Flowerpot is better than a bucket since it has holes at bottom as well). In the PVC trickle tower I would seal the bottom with egg crate or U/G plate and then fill with some lightweight biomedia like prawn trawler netting (free if you use repair discards) or nylon fly screen or similar. Air flow is very important though so dont just pack it full. If you use a high tower or have heavy bioloads, some airstones in bottom of the tower to create counter current aeration is extremely effective. Place the airstones underwater in the tower and the air will have to go upwards against waterflow to escape. If you dont like a vertical post inside your pond, it is easy enough to have it beside the pond too.
You can paint the tower or wrap in aquarium background if you want to make it look less DIY.
Just some ideas for you.