Anna, there are a few threads on here about bustle pads that a quick search should bring up.
A site that I recently found has some simple directions for a bum roll, that may be more Ren Faire, but is almost the same concept. http://www.reddawn.net/costume/bumroll.htm
I guess you could make it biiger depending on how much padding you want.
The pad probably needs to go under your hoop to give a nice sloping line for a smooth silhouette. There are a few pictures out there showing this from the period, or from "period" movies.