Aftermarket plastic for vintage bikes is not as thick or resilient as that which come on the bike originally.
Unfortunately, the companies that produce plastic using the same process as oem (acerbis, ufo, cryca) do not make items for older bikes.
Fortunately, Maier makes the next best thing. As mentioned above, theirs is not as thick or pliable as oem, but theirs is the thickest and most resilient of any manufacturer that is making plastic for vintage bikes.