Decent heads for a monopod and a long lens are a real pain. Ballheads are just too wobbly. Gimbals and similar are too bulky and, while OK for two-dimensional shooting (like sports), connecting direct to the pod is no good for things like aircraft or birds.
What's needed is a decent tilt head. Unfortunately there are only three choices. There's the Manfrotto 234RC2, which is useless for big lenses as it droops horribly. The other two are excellent monopod heads. But both the Acratech Long Lens Head and the Really Right Stuff High Capacity Monopod Head (my preferred option) are extremely expensive.