If they want to fly without control surfaces I would start with those aircraft that are already doing so. Hand gliders have no control surfaces. They maneuver

If they want to fly without control surfaces I would start with those aircraft that are already doing so. Hand gliders have no control surfaces. They maneuver by the pilot shifting their mass. Build a fixed-wing craft with two internal pipes, one for-aft and another left-right. Inside those pipes mount dense weights. Maneuvers are then accomplished by moving that weight along the pipes to create pitch and roll moments. It might not win any dogfights, but a slow and stealthy drone should be able to navigate relatively easily. Yaw would be an issue, but that should be easy enough via differential thrust between a pair of engines. No super advanced tech needed.