[hurch]

Using good old pen and paper or adobe ideas are a good starting point. Don't worry if your sketches look like crayon scribbles by a 3 year old, the act of putting your thoughts down will likely lead to more thoughts/tweaks to the original idea. And the more you do this, the better your design skills will get. After that, and acknowledge that you had said not to actually build it, I'd move to putting together a basic prototype with any parts that you can get your hands on - trade/building supplies/art and craft store/daiso/discount store etc. The benefit of doing this is that once you/potential customers can handle the product, you'll learn more than any visual mockup will provide and the design will likely change again. Going through this cycle several times will lead to a much better design.

List of visual mock up tools (free and paid): http://i.materialise.com/creationcorner http://www.shapeways.com/tutorials/supported-applications