On both my '55 cars, I replaced the original cracked rubber o-rings with duxseal.
Duxseal is a product use in refrigeration seals and other uses. It comes in a glob that looks like silly-putty. You can tear off a portion and roll it out with your hand on a flat surface until it takes the shape of a long worm. I just used my eyes to judge the diameter. Keep rolling till you get a diameter similar to the original o-ring. It can be made in sections, and when you get enough rolled out, just smash the section ends together and roll out round. Place the finished duxseal gasket in the trough of the defrost boot and install the nuts on top. As you tighten the nuts down, it will smash the duxseal into the trough and up against the bottom of the cowl. Ta da!