I agree that interfacing isn't really necessary but if you really feel that you need it just get a soft tricot fusible. Its soft and will drape nicely but gives a bit of body.
I make a lot of costumes (mostly for dogs lol) and I'll give you a hint. Next time buy some of the synthetic felt and make it out of that. One layer, no hemming, no lining, no binding seams. Just cut it out and sew it up. I do it all the time. They're cute as anything and usually look better in the end. If you want to embellish, you can just glue of fuse stuff on. It's so much easier.