For quick prototyping I’ve used clear polyethylene plastic sheeting (sold as vapour barrier for home construction or floor/furniture protection while painting) held together with vapour barrier tape. Both are available at building supply stores (in North America, at least).
Often making things smaller is significantly more difficult because it’s hard to work around small radius curves, so if you use cheap materials and make it full size then it’ll be easier.
Obviously I’m biased, but I would avoid the cheap Alibaba TPU fabrics, especially if you intend to use heat to weld them.