I have only ever used that acrylic ChromaKey paint on solid walls, floor and ceiling but the fabric recommendation was a medium to heavy cotton, so you’d be fine with that.
For outdoor stuff I use pre-dyed cotton fabric suspended from a support frame with sprung clamps. If I need to go wider I use two frames and leave a few feet hanging off the left side of the second frame – this then overlaps the frame uprights where they meet and clamps to the top bar of the first frame, thus hiding the uprights. It forms a vertical join on the fabric but this doesn’t cause any problems with the keying if kept flat (no puckers). If the screen needs to be in an L shape, say to the front and side of your car, the overlap can be swept gently where the two frames meet to form a nice curved corner.
Just in case you didn’t know, be wary of the sun moving in and out of clouds when you shoot – this will give you major headaches later…
