I think you have two options.
You could go to the fabric store or craft store and purchase a block of foam, I cant remember if their were different grades but their should be. The foam that I am thinking of is green and it brakes apart easy.
You can shape the foam with a coarse file, sand paper or a scorch bright pad on a angel grinder. You could shape the foam into the curve you want. Expanding foam is another option, it cost more but you can build the shape you want and then "detail" the shape after the foam dries.
Depending on how much time you want to invest, after you shape the foam you could apply body filler to it and then shape the body filler. While the body filler is still curing you can shape it with a tool called a chess-grader or just sand it after its cured. The smoother your surface is to lay fiber-glass on, will in-turn reflect how good your kick panel turns out.
I am really kind of unclear on your curve but another option is using the card-board from a cereal box, you can cut out a section of the box and then roll the section of card board around a can of pop or something and get the card board to curve, then trim the card-board how you want it and use 2inch masking tape to hold it how you want it and then fiberglass over it.
The problem with using a balloon is that resin can get hot and it can eat thru plastic and pop the balloon.
