I've heard of K2R spot remover and also kitty litter. I used the litter once. I poured it into the cavity and let it sit for days. Whenever I'd walk past the plane I'd shake it a little to move the litter around to soak up more fuel. It worked, but to this day I still have litter rolling around that area. I just can't seem to get it all out.
I did not have K2R or I would have tried that instead.

Whatever you decide to do, make sure you fuel-proof the area after you get it cleaned up.

Good luck!