Check out info on resin casting. You have a mold, mix the resin with a catalyst. Whatever is in the mold gets embedded in plastic. - This link doesn't include info, but it's a reminder to go check out more info on resin casting
Takes a bit of thought but quite easy if you’ve done some machine sewing before BUT remember only sew one square gusset on the liner so can turn it inside out through the unsewn gusset (corner) then hand stitch the second one inside.