One of my best performing water-only soil mixes was plain old FFOF mixed with seabat.
http://www.homedepot.com/p/Greenbelt-Organics-Sea-Bat-Fusion-Guano-2-lb-Organic-Granular-Fertilizer-GB00561P2LBS/204583312
Hell, I bet it wasn't even necessary to mix it; probably could have just done a top dressing since it doesn't need to cook. Not sure about exact measurements, but it was for a 7 gallon pot so probably about half a cup per cubic foot.
Now that I've found a local source of FFOF for $13 (1.5 cu ft), the cost is not prohibitive.
Even though peat moss is cheap ($12 for 3 cu ft at home depot), you've still got to buy all of the other amendments, mix the soil, and let it cook for at least three weeks. Also gotta store all of the leftover stuff for the next round. And there's still the risk that the custom soil mix could have problems, ph for example.
The more I think about it, FFOF looks like a very good option for you.
In your situation, it doesn't look like you are going to use a lot of soil anyway -- and probably don't have room to store a lot of extra crap. If you can get FFOF for less than $20 it's probably worth it.