Sorry Luke. In that case, dig a hole, put a liner in it and let Nature take it's course.
You want to know how to keep koi healthy atr a bare minimum? Geez, for how long? 20 - 30 years? Well that requires a system design approach including a structure at a bare minimum. Even you realized a structure was necessary. Everyone wants to see "miminums" but has yet to realize that there really aren't any "single" minimums. Faster turnover rates can be used to partially compensate for volume but then would require more cleaning maintenance and water changes. The parts of the design are interdependent.
Someone asked about additons within my design The skimmer system (gravity fed) can easily be fit with an additional Nexus as an example. I also mention a stubbed out mid-level for further filter expansion. The initial design is a 2 pump system. Is that necessary? For me it most certainly is. Redundancy is everything. Nothing worse than losing a pump and not being able to get it fixed or replaced in a day or 2 to 3. Redundant system are a safeguard along with daily functionality.
I belive that same person asked about airation? Simple, diffuser drain and with a Nexus or other airated moving bed filter or shower, there is plenty of air and really no "need" for a waterfall in that regard.
Steve