An air plant display should let you care for the plant, not merely hold it upright. Before fixing a tillandsia into a shell, cup or narrow opening, think about how water will drain and how the plant will dry.
Choose an arrangement that can be inspected and, preferably, taken apart without tearing the plant. Avoid forcing it tightly into a space that hides the base.
Test the empty display first
Water the empty holder and see where moisture collects. If a pocket remains wet, reconsider the position of the plant or choose another support.
Identify the tillandsia and use watering guidance appropriate to it and your room. 'No soil' does not mean 'no water', and an attractive glass enclosure does not remove the need for suitable light and air movement.
After watering, check the plant rather than assuming the visible leaves tell the whole story. Water can remain in a hidden crevice after the outside looks dry. Build the display around a care routine you can maintain. A removable plant is easier to inspect than one permanently buried in decoration.
Related reading: Air Plant Care: Watering, Light, Mounting and Flowers.