It wouldn't be too difficult with some suitable blue fabric, some flexible plastic sheet (or cardboard/card-stock) and possibly some poly batting for padding if you want to puff/thicken it out a bit. You don't even have to sew it if you glue the fabric to the plastic where it will be on the inside against your body. Then you can use some velcro to strap it together like on the real thing.

But for $25 I'd go the eBay route in a heartbeat if there was enough time.