inside template to outside template
I'm trying to fabricate something from fiberglass for a boat project, and one step of the process requires that I make a hole with a particular shape.
The "foot" of a marine toilet that I'm working with has an "H"-shape ... actually, if you imagine a wide letter H made of wax that then melted, so that all the corners get rounded -- that's the shape.
I can mount a piece of masonite to the toilet base and chase around it with a flush trim bit to get an H-shaped piece of masonite, which I'll call an "inside template".
What I need is an "outside template" -- something that, when traced with a flush-trim bit, will produce a hole into which the inside template will fit perfectly.
I'm pretty sure that this can be done with something like an oversize bearing, or ... who knows? But I don't actually know much about routers, or what bits/tools/whatever might be available -- in all my years of messing with boat stuff, they haven't come up much. So in explaining how to do this, please don't be afraid to talk down to me, and to explain the tiniest and stupidest detail -- I promise not to take offense.
Can someone point me to a quick tutorial, or just say "That's easy! You simply do X, then Y, then Z and you're done"? Thanks in advance.