An acrylic crystal is flexible and it should be possble to fix it in place without adhesive. The trick is to buy the correct size, or buy a couple and use the one that fits best, using a crystal lifter to fit the crystal if it doesn’t have a tension ring. If it has a tension ring a crystal press will be needed to press it in.
I have a simple test to check if a crystal’s fitted tightly: press hard on the crystal and attempt to twist it, if it moves the next size up is needed.
Paul