Okay, his head has some structure going on. If you don't want to make it full plushie (which would get quite hot), you are going to have to make a structure and put the fabric over it. It won't work just trying to have a fabric hood; the top of his head will sag and flop down limp, you won't be able to get the face details like the eyebrow ridges or the lips, etc. Some people use plaster bandages, some people build with upholstery foam, some use plastic canvas as a base and put craft foam or upholstery foam over that when dimension or smoothness is needed.
You can also install a cooling fan in the head to help reduce the heat of wearing a costume like this.
One thing you can try is getting some strong starch or fabric stiffener. I've seen some impressive stuff that involved only burlap, sewing, and fabric stiffener. I don't know what the result would be using fabric stiffener on bedsheets, though...it's a much finer weave than burlap. Fabric stiffener is basically like a glue or sealer, so using it on bedsheets may make it less breathable and it could be unpleasant against the skin.
This would probably be a good project and question to pose in the Props/Accessories/Armor forum, too. The people over there will generally have a lot more experience with constructs, and they'll provide another realm of feedback that you can consider.
FF9 cosplay? Hit me up!
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Last edited by Evil Bishounen : 11-18-2013 at 10:18 PM.