For both my Napoleonics and 18th Century set-ups, I have used the same command structure. All of my mounted commanders and staff officers are on oval bases, 20mm wide and 30mm deep.
Brigade Commanders are represented by a single figure, such as this of the Duke of Perth. However he later served as a Divisional or Wing Commander, so can be added to a sabot base with an ADC (as below) in that role. He is a Strelets figure.
Divisional (or Wing) Commanders are represented by two figures, mounted onto a 50mm diameter sabot, which can have the commander figure slid in from the front and his staff officer from the back. Here the commander is Sir John Cope (Strelets Swedish Dragoon) accompanied by his Adjutant General Colonel Loudoun (heavily converted Italeri Napoleonic British Hussar, with his pelisse welded into a plaid cloak).
Corps Commanders (effectively Army Commanders in this period) are represented by a 60mm diameter sabot of three figures, such as that of Prince Charles Edward Stuart, Col O’Sullivan and a Standard Bearer. Prince Charles was converted from a Revell mounted 18th Century Prussian (the slim figure looked right), Col O’Sullivan from a Revell SYW Austrian Artillery Driver and the Standard Bearer from an Italeri Napoleonic British Hussar.
A similar sabot is the Duke of Cumberland and his staff. Cumberland’s upper body is that of an Airfix Friar Tuck figure, from their Robin Hood set. I have used that same figure, also heavily converted, as Napoleon and the Zulu chief Cetshwayo. Cumberland’s ADC (Col Cathcart) and his Standard Bearer are both Strelets Swedish Dragoons. I used to paint my own flags, but now find suitable ones on-line, modify them to show both the obverse and reverse of the flag, then print them out. Cumberland’s is his personal Royal Standard.
I do have some foot commanders, mainly artillery, such at Lt Col Whitford shown here (he was 5th Marines, but commanded the British Artillery at Prestonpans, since he had experience of Naval gunnery). I model all of my artillery officers looking through telescopes. He is an IMEX AWI British Artillery figure. He is on a round 20mm base formed by sticking a 2 cent Euro coin onto a circle of card, glueing the figure on top and then welding plastic all around the base. A British 1p coin is about the same size, and worth less, but I would not dream of defacing that. A few of my Jacobite Brigade Commanders are on foot, as are several British fortress commanders.
For the War of Austrian Sucession I may make some 70mm diameter Army Command Sabots to hold four figures, as I planned to do for the Napoleonic Wars, but never got around to.