What an interesting concept. No rules? Nothing about how much fuel?

This is a first off sort of concept idea but I'd start with a large sailplane like wing running a higher lift airfoil that still offers reasonably low drag. Some of the high lift Selig offerings like the 1223 would be good. Worth looking into the drag thing though. You want to use as small an engine as you can get away with so it's probably worth trading off some lift for lower drag.

The model for my concept would be loosely based on something like a 100 inch wingspan polyhedral glider with a big fat guppy shaped fuselage to hold the fuel. The main pod of the fuselage should be shaped using a proper airfoil to offer lower drag. And to gild the lilly the fuselage would be "cambered" so the mean camber line followed the airflow entering the leading edge and the downwash off the trailing edge.

Why polyhedral? Simple. You can then trim the model to fly as a free flight with turn trimmed into the flight path. This would reduce the pilot's workload to using minor throttle changes to trim for altitude and minor rudder and elevator trim changes to trim the model to keep it "on the step" as the fuel burns off. The pilot would only disturb the circling flight to return upwind occasionally and would then release the controls to let the model return to circling flight.

Engine mods. Change the carb to one off a smaller displacement engine. How small? You still need enough to get the model airborne with a full load but only just. Full power should offer a very lazy climb but it should be enough that you can deal with wind turbulence. Change over to gasoline and spark ignition. Gasoline has a lower stoichiometric mixture ratio and more BTU's per pound than glow fuel so you use less gas per minute to achieve the same power. It's been good enough for Maynard Hill to keep the duration record so it should be good enough for you if the rules allow gas. 4 strokes have overlap between the intake and exhaust functions so you loose less fresh fuel out the exhaust. So go for a 4 stroke spark conversion with the carb off the next size down engine.