Dude, nice work! Especially considering it's a prototype and sewn by
hand. I just wouldn't have the patience for that.
A nice and clean way to make a sleeve for that plastic piece is to sew two pieces of fabric together along the long edges and have that be your front panel. Then sew one of the ends of the panel as your very last seam of the project and put the plastic in before you sew it up. Harder to change it out later if you decide to, but that's what your prototype is for, right?