I've seen photos of a sponge bob done. Wouldn't be that hard if you can get to a fabric store or target and get some large Matress top Foam.
Foam cuts pretty easy, so you could cut out the dimples to look like a sponge. Could either go 2D or more 3D with it, by either putting two thin halfs together, glued on the inside- just around the edges would be a good start. Just leave openings for arms and legs... or maybe the bottom section completely open to get into it (pulling it over like a big shirt).
Option B: glue foam onto a larger cardboard box on the outsides of it for a more square 3D look.
paint on or add to it the eyes, features and add the top part of the pants and a belt to the lower section... and then just wear same color pants so the costume and pants blend together.
I think it would be pretty easy to pull off. A foam latex paint over the whole thing should seal it pretty well.