A method taught by Jack Brooks that he has seen on several originals: Make your barrel underlugs 1/8" tall, or 1/8" proud of the barrel when installed more specifically. Inlet the barrel lugs, then drill a small pilot hole straight down through the center of of your barrel lug inlet. This will locate the center of your ramrod channel. Gouge, scrape or plane your ramrod channel until you break through to the lug inlets, then thin as much as possible. This will make the thinnest practical web of wood between the barrel inlet and ramrod channel, which also closely follows the barrel contour. Makes for an incredibly slender gun.
Curtis