What about using two layers of plastic (like laminating film)? You bond them together around the edges and in multiple places in the middle like spot welds (so it doesn't balloon out too much). Get an inlet/outlet and you're set to attach it to whatever you want.
We use something similar as a cooler at work. They call it a pillow plate, but its made of steel. It looks like this:
The only other thing I've seen besides the very expensive ones was someone here was making a liquid cooled seat where he was embedding the tubes into his driver's seat.