power supply - Physically lightest topology for SMPS?

I'm being vague about part of my project, but please ask me to fill in critical details I'm missing.

I need to power a 24V @ 20A device for 5 hours (one time), within a weight budget of 700 grams, at a distance of 500ft from the ground. A 500ft tower is impractical in this instance. Nobody will die if the device fails, and since it's a hobby project I am OK with some serious liberties in terms of normal safety devices to drive the weight down.

The highest voltage I can realistically attain is 310-340 volts DC (rectified & smoothed 220), which is then raised via a pair of 26awg cables 500ft in the air, and transformed back into 24V @ 20A. My initial plan is to hack apart a COTS power supply, and strip out the AC rectification side (doing that on the ground), remove as much cooling as I can (because I will have ample airflow), and other hacks -- but I wonder if another topology can be made simpler and still handle a 15:1 reduction ratio. I'm assuming a fair amount of weight is being taken up with the full transformer inside the power supply, even though it is a high-frequency design.

I can live with high ripple and poor regulation (0.5v P-P, 10% regulation), I don't need mains isolation. Am I better off chaining 3 buck regulators? Should I go for a custom-wound transformer and a flyback topology?