Casual creators are creativity support tools intended to be fun and easy to use for exploratory creation. We have built casual creators called fluidic game designers, which support exploratory game design directly on mobile devices. These work by encoding games in a parameterised design space and enabling player/designers to create new games by varying the parameters in a design interface and seeing changes in their design instantly. A question largely unanswered by existing work is how to choose a suitable parameter space. We describe a methodology for specifying and implementing parameterised design spaces for casual creators, a context that requires balancing a large and expressive space against a manageable and fun user interface. This methodology was derived through investigating and generalising how the parameter spaces for three fluidic games were conceived. It suggests an iterative process whereby parameters are sourced in seven different ways, within a dynamic of incremental expansion and contraction of the parameter spaces.