Base for refrigerator wine shelf
Tempered glass (might be costly, might not; but a good and common solution seen in fridges, easy to clean, won't harbor mold, etc.)

Plastic would work but will eventually get scratched up somewhat, but is also a common refrigerator interior suface.

Wire mesh (fairly heavy, stiff) works and permits airflow. Stainless steel or powder-coated to prevent corrosion. Again, a common shelf material in this application.

Have you verified that the "wine fridge" will actually operate properly at the temperature range you require? many are optimized for "cellar temperatures" in the mid-50's F (10 C) and don't do well running at normal refrigerator temperatures in the lower 30's (1-3 C)