Not sure why this feels like it should be different than on a straight barrel. It is exactly the same, the thickness of the wood beside the barrel. The difference is, the barrel is tapering and flaring. But at any given point along the barrel, the width of the wood is the same as it would be on a stock for a straight barrel.
In the fore-arm area from the lock to the entry thimble, an egg or elliptical shape is typical. On a swamped barrel this part of the stock tapers in width along with the barrel, of course. Top to bottom this egg shape may taper slightly from the guard to the entry thimble.
Then there is a transition to a fore-end shape at the rear thimble, to the muzzle. This can have different shapes depending on the style of gun, but again, at any given point, it’s the same as on a straight barreled gun. It just has to flare as the barrel flares.