Physically lightest topology for SMPS?
What about the MeanWell SDR-480P-24 ? Maybe gut the PCB and chuck the chassis... the PSU feels pretty light including the metal chassis. It's fanless and seems to have a pretty good efficiency - making me wonder if the secondary side is sync-rectified or conventional schottky-based.

If you insist on DIY, I would say the forward concept needs less mass in the inductor/transformer core than flyback, and the more modern FET generations are more efficient = need less heatsink surface (and mass). The recent TrenchFET/FinFET generations have gradually less and less gate capacity and RdsON. Hence the most recent PSU generations run gradually cooler and are lighter. Synchronous rectification on the secondary side adds to that effect, although at 24 V I'm wondering if the effect is still noticeable, compared to a Schottky diode. As for ripple voltage... you can only stretch the secondary elyts a little, not 200%. They have a problem with internal heating (due to RdsON) and relatively little heat-dissipating surface, no amount of forced airflow will quite help. Solid Polymer are available up to 25-35 V, they're light and have a huge permitted ripple current (at around 100 kHz optimum switching frequency). So I don't think ripple voltage will be an issue, if you stick to the elyts' permitted ripple current.

If you need 5 hours continuous runtime, thermal capacity (inertia) alone won't allow you to undercut heatsink surfaces/masses (compared e.g. to a UPS designed to last for 10 minutes). Plenty of forced airflow is a different story though :-)