Not desperately hard to build a vertical-drop fixed, just keep in mind two things 1)it is a hack 2)it is a hack.
Not too hard at all to find a magic ratio. Have a half link and a couple of rings and cogs handy and try them out, or try one of the online calculators. They may even work, who knows?
Be prepared that the chain will stretch... then you need to find a game plan. Mine was to pick up a file and attack both the dropout and the axle. Did I tell you it is a hack?